Abstract
1. 1. Tilapia were acclimated to different concentrations of urea and mannitol. 2. 2. The fish treated with 310 mOsm urea survived for about 10 days. Those treated with 350 mOsm mannitol survived for longer. 3. 3. Urea penetrated quickly into the animal. The internal concentration was about 2 3 of the external concentration after 10 days. This suggests a higher excretion rate than uptake rate. 4. 4. Both treatments resulted in similar changes. The dehydration was more clearly indicated in tilapia by an increase of plasma protein, lowered intracellular space of muscle cells and Na-content of muscle cells. These changes were well regulated during both treatments. 5. 5. The regulated changes were maximal one day after the start of acclimation. 6.li] The stress effects of these treatments were seen at changes which occurred later than one day after start of acclimation: decrease of Na-K-ATPase activity, loss of plasma Na during mannitol influence. increase of plasma glucose and decrease of liver and muscle glycogen during both treatments. 6. 7. Differences between stenohaline carp and euryhaline tilapia were discussed.
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