urea-N Excretion Research Articles

Biochemical indicators of condition, nutrition and nitrogen excretion in caribou

Urinary urea nitrogen to creatinine ratios, urinary Nt-methylhistidine to creatinine ratios, serum urea nitrogen concentrations (SUN mg/dl), and serum Nt-methylhistidine concentrations were compared with physical measures of body composition in adult female barren-ground caribou (Rangifer tarandus groenlandicus) from the Bathurst and Southampton Island herds during late winter. Body weight and UUC were used to estimate urinary urea nitrogen (urea-N) excretion in free ranging caribou. Only mean UUC reflected differences in fat reserves between populations. None of the biochemical indicators were directly related to body composition. However, elevated UUC were only observed in caribou with depleted fat reserves as demonstrated by low kidney fat index (KFK40) and/or reduced femur marrow fat (FMF<80). UUC greater than 0.25 were indicative of undernourished animals with depleted fat reserves. SUN and UN -MHC showed no clear relationship with fat reserves. The mean estimated daily urea-N excretion for adult female caribou in late winter was extremely low (0.11+0.01SE g urea-N/day, n=76, range=0.011-0.510). The results of my study suggest that UUC can be used to detect nutritionally stressed caribou with depleted fat reserves on lichen winter ranges.

Induction of ureogenesis in perfused liver of a freshwater teleost, Heteropneustes fossilis, infused with different concentrations of ammonium chloride

The induction pattern of urea cycle enzymes and the rate of urea-N excretion were studied with relation to ammonia load in the perfused liver of a freshwater ammoniotelic teleost, Heteropneustes fossilis, when infused with different concentrations of ammonium chloride for 60 min. Both urea-N excretion and uptake of ammonia by the perfused liver were found to be a saturable process. The V max of urea-N excretion (0.45 μmol/g liver/min) was obtained at ammonium chloride addition of 1.18 μmol/g liver/min. The maximum induction of carbamyl phosphate synthetase (ammonia dependent), ∼200%, and of ornithine transcarbamylase, ∼120%, was seen by the addition of 0.58 μmol/g liver/min, and for argininosuccinate synthetase and argininosuccinate lyase of ∼150% and 115%, respectively, by the addition of 2.8 μmol/g liver/min of ammonium chloride. However, arginase activity did not alter in any of the concentrations of ammonium chloride added. An increase of ammonia load of 3–5 μmol/g wet wt from the physiological level in the perfused liver was sufficient to initiate and to cause maximum induction of most of the urea cycle enzymes activitty. These results further confirm the capacity of transition from ammoniotelism to ureotelism in this unique freshwater air-breathing teleost to tolerate a very high ambient ammonia.

Responses of oxygen consumption, Ammonia-N excretion and Urea-N excretion of Penaeus chinensis exposed to ambient ammonia at different salinity and pH levels

Oxygen consumption (O 2 mg g −1 h −1) of Penaeus chinensis juveniles (0.76 ± 0.05 g) increased with increased concentration of ambient ammonia-N in the range of 0–10 mg 1 −1, but decreased with increased salinity level in the range of 10–30 p.s.u. (practical salinity units) and pH level in the range of 7.0–8.5. Urea-N excretion and nitrite-N excretion of shrimp increased with increased salinity, pH and ambient ammonia-N levels. Whereas, ammonia-N excretion of shrimp decreased with increased salinity, pH and ambient ammonia-N levels. Ammonia-N uptake occurred when shrimp were exposed to ambient ammonia-N at 5 mg 1 −1 or greater at every salinity and pH level tested. Shrimp at an increased salinity level, and at an increased concentration of ambient ammonia-N enhanced their urea-N excretion, an effect that was more pronounced at high pH than at low pH.

Nitrogenous excretion in juvenile turbot, Scophthalmus maximus (L.), under controlled conditions

Laboratory experiments on juvenile turbot. Scophthalmus maximus (L.), were carried out under controlled temperature (12°C) and feeding regimes in a flow-through system. Monitoring of total ammonia nitrogen (TAN) and urea nitrogen (Urea-N) was performed through continuous sampling of the effluent sea water. The effects of ingested nitrogen levels on TAN and Urea-N daily and hourly excretion rates were studied. Strong relationships were found between ingested nitrogen, and both TAN and Urea-N excretion for both daily and hourly maximum excretion rates. Turbot showed a low metabolic activity, confirmed by low excretion levels. Daily patterns for TAN and Urea-N production were different, suggesting specific physiological phenomenon for urea excretion mechanisms. The question of whether turbot are partly ureotelic or ureogenic is underlined.

Pulsatile urea excretion in the ureagenic toadfish Opsanus beta: an analysis of rates and routes

This study focused on the rates and routes of urea-N and ammonia-N excretion in the ureagenic toadfish and on the possibility that urea-N excretion occurs in pulses. Experimental approaches included the following: confinement in small individual containers with automated hourly sampling of water to follow temporal excretion patterns; divided chambers to separate excretion from the anterior and posterior parts of the fish; collection of urine and rectal fluid via chronic indwelling catheters; and gavage with [14C]-labelled polyethylene glycol 4000 to detect regurgitation of gastrointestinal fluids. When a standardized 'crowding' pre-treatment was employed to induce ureotelic behaviour, the fish exhibited significant elevations in the activity of glutamine synthetase in liver, kidney and gills, elevated plasma and bile urea-N levels, but unchanged ammonia-N and urea-N levels in most other body fluids. Unencumbered ureotelic fish confined in small containers excreted 82 % of their waste-N as urea-N and 18 % as ammonia-N; almost all (94 %) of this urea-N excretion occurred in a single pulse of less than 3 h duration about once every 24 h. This daily pulse did not occur by regurgitation of gut fluids, by excretion through prominent pores behind the pectoral fins or by discharge of rectal fluid or urine. Intestinal and urinary excretion accounted for less than 10 % of whole-body urea-N excretion and a negligible fraction of ammonia-N excretion. Pulsatile urea-N excretion occurred at the head end across the gills and/or body surface. Ammonia-N excretion, which was not pulsatile, also occurred largely through the head end. However, once the toadfish had been placed in divided chambers, urea-N excretion became continuous rather than pulsatile, and ammonia-N excretion increased greatly. A severe stress response was indicated by high levels of plasma cortisol, and the skin, which lacks scales, became a significant route of both ammonia-N and urea-N excretion. We speculate that the normal adaptive significance is that ureotelism facilitates cryptic behaviour, allowing the toadfish to virtually eliminate N-waste excretion during long periods while it remains sheltered in burrows. However, during severe stress, the effects of extremely high cortisol levels overwhelm the ammonia and urea retention mechanisms, and both substances leak across the general body surface.

Ammonia and urea metabolism in relation to gill function and acid-base balance in a marine elasmobranch, the spiny dogfish (Squalus acanthias)

Nitrogenous waste excretion in resting dogfish occurred largely (>90 %) as urea-nitrogen (urea-N) efflux across the gills, with a very small urea efflux via the kidney. Ammonia excretion, almost entirely at the gills, accounted for less than 3 % of total nitrogen excretion. Given the extremely high blood urea levels (approximately 640 mmol-N l-1) 'retained' for osmoregulation, and blood ammonia levels (approximately 80 µmol-N l-1) comparable to those of teleosts, the gills of resting dogfish were exceptionally impermeable to both urea and ammonia. Experiments investigated the origins of these low permeabilities and the responses of urea-N and ammonia-N excretion and acid­base status to 6 h infusions with iso-osmotic solutions of NaCl (control), NH4Cl, NaHCO3, urea and its analogues thiourea and acetamide. NaCl had no effects, whereas NH4Cl loading caused intense acidosis and marked elevation of acidic equivalent, ammonia-N and urea-N excretion rates, the latter despite unchanged blood levels of urea-N. Apparent branchial ammonia permeability increased greatly. Acidosis resulted from both stimulated urea production and branchial NH3 loss, the former making the larger contribution. NaHCO3 loading caused intense alkalosis, a marked elevation of basic equivalent excretion and a moderate stimulation of urea-N excretion. Blood urea-N levels were again unchanged. Infusion of urea itself raised blood urea-N levels, but initially reduced branchial urea-N excretion. Acetamide and thiourea infusions both moderately elevated branchial urea-N excretion. We suggest that the low ammonia permeability may arise metabolically from an ammonia scavenging system in the gills, that a 'back-transport' mechanism in the gills may contribute to the low urea permeability, and that the dissociation between blood urea-N levels and excretion rates may reflect urea production at extrahepatic sites. These studies demonstrate that urea synthesis in the dogfish is linked more to nitrogen availability than to acid­base status.

Effects of dietary protein/energy ratio, ration size, dietary energy source and water temperature on nitrogen excretion in rainbow trout

Postprandial nitrogen excretion was measured on groups of rainbow trout of mean body weight (MBW) 35g, fed, from maintenance to near satiation, diets containing graded levels of digestible energy (14 to 20 kJ/g dry matter) with digestible protein (DP)/digestible energy (DE) ratios of 18 and 23 mg/kJ at each protein level (DP: 32.5 or 37% dry matter). Irrespective of the diet and the ration size, urea-N excretion amounted to 15-16% of ammonia-N excretion. The higher the dietary DP/DE ratio, the greater was the amount of ammonia excreted. At high ration levels (1.5% and 2% of MBW/d), N excretion was 39-40% of digestible N intake for diets with a DP/DE ratio of 18 mg/kJ and 44% of digestible N intake for diets with a DP/DE ratio of 23 mg/kJ irrespective of the dietary protein level. At lower ration levels, N excretion as percent of N intake was higher than at higher feeding levels. Another experiment was performed to compare the effect of dietary lipid and digestible carbohydrate on ammonia excretion in rainbow trout reared at two water temperatures (8°C and 18°C). Neither water temperature nor dietary sources of non-protein energy had any significant effect on N excretion expressed as proportion of digestible N intake.

Accumulation of urea in the haemolymph and ammonia excretion of Penaeus japonicus exposed to ambient nitrite

Penaeus japonicus (15.7 ± 1.4 g) were exposed individually in 30 ppt seawater to 0.01 (control), 5, 10, 20 and 50 mg/l nitrite-N for 24 hr. Haemolymph ammonia, urea, nitrite and whole shrimp ammonia-N excretion and nitrite-N uptake were then determined. Ammonia excretion of P. japonicus increased with increased ambient nitrite, and with a concomitant decrease of haemolymph ammonia, as occurring increased concentrations of nitrite. Concentrations of nitrite-N and urea-N in the haemolymph of shrimp increased with increased ambient nitrite-N. However, no urea-N excretion was observed for shrimp exposed to any nitrite treatments.

Effects of dietary protein/energy ratio, ration size, dietary energy source and water temperature on nitrogen excretion in rainbow trout

Postprandial nitrogen excretion was measured on groups of rainbow trout of mean body weight (MBW) 35g, fed, from maintenance to near satiation, diets containing graded levels of digestible energy (14 to 20 kJ/g dry matter) with digestible protein (DP)/digestible energy (DE) ratios of 18 and 23 mg/kJ at each protein level (DP: 32.5 or 37% dry matter). Irrespective of the diet and the ration size, urea-N excretion amounted to 15-16% of ammonia-N excretion. The higher the dietary DP/DE ratio, the greater was the amount of ammonia excreted. At high ration levels (1.5% and 2% of MBW/d), N excretion was 39–40% of digestible N intake for diets with a DP/DE ratio of 18 mg/kJ and 44% of digestible N intake for diets with a DP/DE ratio of 23 mg/kJ irrespective of the dietary protein level. At lower ration levels, N excretion as percent of N intake was higher than at higher feeding levels.Another experiment was performed to compare the effect of dietary lipid and digestible carbohydrate on ammonia excretion in rainbow trout reared at two water temperatures (8°C and 18°C). Neither water temperature nor dietary sources of non-protein energy had any significant effect on N excretion expressed as proportion of digestible N intake.

Partitioning of Nitrogen Excretion between Urea and Ammonia and between Skin and Kidneys in the Aquatic CaecilianTyphlonectes natans

Nitrogen excretion was studied in the aquatic caecilian Typhlonectes natans by analyzing samples of the bathing medium over an 8-h period for total ammonia ($T_{amm}$: NH₃ + $NH_{4}^{+}$) and urea. Total ammonia was excreted at a rate of 0.35 μM · 10g⁻¹ · h⁻¹, while urea was excreted at a rate of 0.19 μM · 10g⁻¹ · h⁻¹. When excretion is expressed in molar equivalents of nitrogen, ammonia-N excretion was 0.35 μatm N · 10g⁻¹ · h⁻¹ while urea-N excretion was 0.38 μatm N · 10 g⁻¹ · h⁻¹. These latter values uwere not significantly different. Plasma $T_{amm}$ concentration was 1.3 mM, while plasma urea concentration was 4.4 mM Urine $T_{amm}$ concentration was 3.8 mM, while urine urea concentration was 2.7 mM. Partitioning $T_{amm}$ excretion between skin and kidneys found 1.11 μM · 10 g⁻¹ · h⁻¹ ammonia excreted across the skin. The kidneys contributed 0.09 μM · 10 g⁻¹ h⁻¹, or less than 8%. Skin urea excretion rate was 0.21 μM · 10 g⁻¹ · h⁻¹, while the kidneys accounted for 0.08 μM · 10 g⁻¹ · h⁻¹, or less than 28%.

Comparison of a questionnaire to estimate dietary sodium and protein intake to measured urinary sodium and urea-nitrogen excretion

Daily urinary excretion rates of sodium (Na) and of urea-N should accurately reflect daily dietary Na and protein intake in the steady state. Self-administered dietary questionnaires are often used to assess Na and protein intake but their accuracy in relation to measured urine composition is not known. We compared estimates of dietary Na and protein intake based on a self-administered questionnaire, analyzed using Nutritionist-IV software, to the measured mean daily urine Na and urea-N excretion rates in 3 24-hour urines collected by each of 94 healthy adults eating their usual home diets. We studied 50 women and 44 men, ages 20–70 years, BMI 25.2 ± 3.7 SD kg/m2. The urines were accurately collected as judged by their creatinine content in relation to body weight (r=0.65) and to predicted individual daily creatinine excretion based on sex, age and weight using the Cockcroft & Gault formulas (r=0.80). Measured urinary Na excretion, mean 156 and range 47 to 322 mmol/day, was greater than and not significantly related to estimated Na intake based on the questionnaire, mean 114 and range 28 to 239 mmol/day; r=0.14. Similarly, dietary protein intake; estimated as ((Urea-N,g/day)+(Weight,kg∗0.032))∗ (6.25), mean 78 and range 36 to 136 g/day, was less than and not closely related to protein intake based on the questionnaire, mean 95 and range 27 to 206 g/day; r=0.20. Both daily Na (r=0.55) and daily protein intake based on urinary urea-N and weight (r=0.76) were related to daily urinary creatinine excretion. Additionally, urinary potassium (K) excretion, mean 67 and range 24 to 125 mmol/day, was less (as expected because of fecal K excretion) but more closely related to K intake estimated by the questionnaire, mean 94 and range 37 to 213 mmol.day; r=0.42. However, urinary K excretion was not so closely related to daily urinary creatinine excretion, r=0.45, as were Na and urea-N. We conclude: 1) the dietary questionnaire underestimates dietary Na intake, presumably because the questionnaire does not take account of intersubject variation in NaCl added during cooking; 2) the dietary questionnaire overestimates protein intake but the basis for this overestimation is not apparent; 3) daily urinary K excretion in accurately collected urines may provide a useful estimate of dietary K intake.

Evaluation of protein intake by dietary diaries and urea-N excretion in children with chronic renal failure

Evaluation of protein intake by dietary diaries and urea-N excretion in children with chronic renal failure

Nitrogen excretion and changes of hemocyanin, protein and free amino acid levels in the hemolymph of Penaeus monodon exposed to different concentrations of ambient ammonia-N at different salinity levels

Penaeus monodon (22.064 * 2.263 g) were exposed individually at 10, 20 and 30% salinity to 0 (control), 1, 5, 10 and 20 mg 1-' ammonia-N for 24 h. Changes in hemocyanin, protein and free amino acid levels, and whole shrimp ammonia-N excretion, urea-N excretion and dissolved organic N (DON) excretion were determined. Ammonia-N excretion decreased with increased salinity and with increased ambient ammonia-N. Ammonia-N accounted for 56.87. 70.43 and 78.13 % of total nitrogen excreted by P. monodon in 30, 20 and 10% respectively. DON increased to 67.26, 78.79 and 86.1% of total nitrogen excreted by the shrimp exposed to 20 mg 1-' ammonia-N in 30, 20 and 10°A, respectively. Net ammonia-N uptake occurred as shrimp were exposed to ambient ammonia-N greater than 5 mg 1 I at all salinity levels tested. Hemocyanin and protein levels decreased, whereas hemolyrnph ammonia, urea, total free amino acid levels and taurine level increased, with increased ambient ammonia-N. Following exposure to ambient ammonia-N, P. rnonodon changed its excretory pattern, accumulation of hemolymph ammonia, and catabolism of hemocyanin and protein to balance osmoregulation.

A rapid method for determination of hepatic amino nitrogen to urea nitrogen conversion ('the Functional Hepatic Nitrogen Clearance').

The Functional Hepatic Nitrogen Clearance (FHNC) is a measure of the functional liver mass as to conversion of amino-N to urea-N. FHNC is the slope of the linear regression of multiple samples (10-20) of urea-N synthesis rates (UNSR) on blood alpha-amino-N concentrations (alpha-AN) during infusion of amino acids. UNSR is measured as urinary urea-N excretion rate corrected for accumulation in total body water (TBW) and loss in gut. A simplified method which estimates FHNC from only two samples of UNSR and alpha-AN was developed. Urine was collected in two hourly intervals: before infusion of alanine, and from 2 to 3 h after start of alanine infusion. Blood-urea-N and alpha-amino-N was measured at the beginning and at the end of each urine sampling interval. TBW was estimated from a nomogram, and gut loss of urea was assigned a fixed value (14%). The two-sample FHNC was calculated as delta UNSR (mmol h-1)/delta mean alpha-AN (mmol l-1). Linear regression analysis of the two-sample estimates of FHNC on the 'true' multiple-sample values of FHNC in an independent population of control and cirrhotic subjects showed the two-sample estimates to be closely related with values of the multiple-sample method, the regression equation being: two-sample FHNC = -0.24 + 0.99 x multiple-sample FHNC, r2 = 0.98. A close relationship was also obtained when cirrhotic patients were considered alone: two-sample FHNC = 0.01 + 0.94 x multiple-sample FHNC, r2 = 0.98.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemolymph PCO 2, hemocyanin, protein levels and urea excretions of Penaeus monodon exposed to ambient ammonia

Subadult Penaeus monodon (21.985 ± 3.546 g) were exposed individually in 30 ppt seawater to 0.060 (control), 0.994, 4.631, 9.593 and 20.728 mg/l ammonia-N for 24 h. Hemolymph pH, PO 2, PCO 2, hemocyanin and protein levels, and whole animal ammonia-N, urea-N and nitrite-N excretions were determined. Urea-N excretion and nitrite-N excretion increased, whereas ammonia-N excretion and hemolymph PCO 2 level decreased with increased ambient ammonia-N. P. monodon exposed to ambient ammonia-N greater than 4.631 mg/l, net ammonia-N uptake occurred, and caused significant reduction of hemocyanin and hemolymph protein and their ratio. In addition, there was a shift from ammonotelic to ureotelic excretory pattern of shrimps following exposure to ambient ammonia-N greater than 4.631 mg/l.

Haemolymph osmolality, acid-base balance and shift of ammonotelic to ureotelic excretory pattern of Penaeus japonicus exposed to ambient ammonia

Abstract 1. Urea-N and nitrite-N excretions of Penaeus japonicus (11.70 ± 2.30 g) increased, but ammonia-N excretion decreased with increased ambient ammonia-N, as they were exposed individually to 0.055 (control), 1.011, 5.121, 10.032 and 20.335 mg/l ammonia-N after 24 hr. 2. Haemolymph pH, PCO2 and osmolality of shrimp displayed an inverse linear relationship with concentration of ambient ammonia-N. 3. Shrimp exposed to 10.032 mg/l ammonia-N after 24 hr significantly affected (P 4. Accumulated ammonia-N in the haemolymph of shrimp following exposure to ambient ammonia-N at 5.121 mg/l or greater caused a shift from an ammonotelic to a ureotelic pattern, and may cause catabolism of hemocyanin and protein to free amino acids to balance osmoregulation.

Influence of Dietary Protein and Lipid on Nitrogen and Energy Losses in Lake Trout,Salvelinus namaycush

Nitrogen and energy loss in fecal and nonfecal excretions by lake trout, Salvelinus namaycush (171–268 g), varied directly with dietary protein (40 and 50% dry weight) and inversely with lipid (10 and 20%) among four test diets. Diets high in lipid increased dry matter digestibility resulting in reduced nitrogen and energy loss in feces. NH3-N constituted 74.0–93.0% of total N excreted in fasted (4–12 d) and fed trout, while urea-N constituted 4.6–17.8% of total N. Excretion of urea-N was high in fish fed the high-lipid diet, particularly when dietary protein was also high. Total N and NH3-N excretion rose to peak values between 4 and 12 h after feeding and thereafter declined to prefeeding levels by 24 h. Urea-N excretion rates varied significantly following ingestion of all but one (50% protein, 20% lipid) test diet, reaching peak values between 4 and 12 h. In fasted fish, total N and NH3-N excretions were significantly low and failed to show a daily pattern; the urea-N excretion rate was significantly high compared with fish fed diets 1, 2, and 3 and followed a daily pattern of change. On average, a larger proportion (13–25%) of food energy is lost through nitrogenous excretion than in the feces (8–22%).

Protein utilization responses of growing pigs to supplemental lysine as liquid versus crystalline form in barley-barley protein diet

The study examined the influence of increasing dietary supplements of liquid compared to crystalline lysine on nitrogen (N) balance and protein utilization in growing pigs (30-100 kg LW) in a 8*8 Latin square. Eight isonitrogenous diets with 160 g CP/kg were formulated from barley and barley protein supplemented with the two lysine sources at levels of none, 1, 2 and 3 g/kg to provide 5.5, 6.5, 7.5 and 8.5 g/kg total dietary lysine. Barley-SBM served as a contol diet. The dietary lysine concentration had substantial effect on N retention and metabolism. The lysine supplements to the cereal-based diet with a low lysine content, 5.5 g/kg, significantly improved N-retention, decreased urinary-N excretion and enhanced protein utilization and daily gain. There was a linear response of N-retention, urinary-N and urea-N excretion and apparent biological value (P

Nitrogenous waste excretion and accumulation of urea and ammonia inChalcalburnus tarichi (Cyprinidae), endemic to the extremely alkaline Lake Van (Eastern Turkey)

The endemic, anadromous cyprinidChalcalburnus tarichi is the only fish species known to occur in alkaline Lake Van (Eastern Anatolia, Turkey). EightC. tarichi were maintained individually in Lake Van water (17 - 19°C; pH 9.8; 153 mEq·I(-1) total alkalinity; 22‰ total salinity) and tank water samples analyzed for 24 h in 2 to 4 h intervals. At zero time, < 1µM ammonia was present and urea was undetectable in the tank water; at 24 h, total ammonia and urea made up 114±32 and 35±25µM, respectively. Over the experimental period, ammonia-N and urea-N excretion averaged 1041±494 and 607±169μmoles·kg(-1) fish·h(-1), respectively. The extent of urea excretion was highly variable between specimens. Uric acid excretion was not detectable.Urea was present at high concentrations in all tissues and plasma (25 - 35μmoles·g(-1)·ml(-1)) of freshly caughtC. tarichi; total ammonia content of the tissues was by a factor of 1.9 (liver) to 3.0 (brain) lower. High arginase activity (2.4±0.2 U·min(-1)·g(-1)) was detected in the liver ofC. tarichi but ornithine carbamoylphosphate transferase, a key enzyme of the ornithine-urea-cycle, was absent. Ureagenesis is likely through degradation of arginine and/or uricolysis. High glutamine synthetase activity (11±0.6 U·min(-1)·g(-1)) and low ammonia content in brain suggest that, like other teleosts,C. tarichi has an efficient ammonia detoxification in the brain, but in no other tissue.Nitrogenous waste excretion at alkaline pH is discussed. The ability ofC. tarichi to excrete high levels of ammonia at extremely alkaline pH is unique among teleosts studied so far. The mechanism of ammonia excretion under Lake Van conditions remains to be elucidated.

Nitrogenous Excretion of Juvenile and Young Japanese Flounder.

Measurements of the rate of nitrogenous excretion were carried out on fed and starved Japanese flounder of 1.5 to 6.5g (juvenile), and 15 to 91g (young) in body weight at 20°C. Ammonia-N and urea-N excretion of the starved juvenile flounder were 0.76±0.20mg-N and 0.09±0.03mg-N/100g/h, and those of the young flounder were 0.23±0.05mg-N and 0.07±0.03mg-N/100g/h, respectively. Immediately after feeding, the rates of ammonia-N excretion per hour of both juvenile and young flounder became 2 to 3 times higher than those of the starved fish, and these elevated rates continued for more than 12h. In contrast to ammonia-N excretion, the rate of urea-N excretion per hour of the fed fish showed a major peak during the period of 6-12 or 12-24h after feeding. In juvenile and young Japanese flounder fed on a pelleted diet containing 7.5% nitrogen at average rations of 3.0 and 1.3%, respectively, it is estimated that 23 and 21% of the consumed nitrogen was excreted as ammonia-N, 3 and 4% as urea-N, respectively, and about 13% as faeces-N within 24h after feeding.