Summary Salt and water are the essential requirements for effective resuscitation of a patient with extensive burns. The optimum sodium load is of the order of 0.5 mmol/kg body weight/% burn, and the total volume of fluid (excluding the replacement of any large evaporative losses) is likely to be between 2 and 4 ml/kg body weight/% burn. Sodium-free water, given to replace any extrarenal losses, should be limited to an amount sufficient to prevent marked hypernatraemia. The priorities in practical management are the establishment of a reliable intravenous line and the planned administration of a suitable salt-containing fluid, with the aim of maintaining vital organ function at the least immediate or delayed physiological cost. The effectiveness of the fluid therapy in correcting existing deficits and in keeping pace with continuing losses should be reviewed at frequent intervals, bearing in mind that the rate of loss decreases with time. Usually, at least one half of the total fluid requirement will have been given in the first 12h, and periodic adjustments to the rate of infusion should be made in response to information gained by monitoring the patient's condition. Urine produced during the first 24h following extensive burns is usually concentrated, i.e. osmolarity between 500 and 1000 mOsm/l and water diuresis, i.e. osmolarity less than 300 mOsm/l, is rare. An hourly output of between 0.5 and 1.0 ml/kg body weight (using 12.5 kg as a minimum weight for infants) of concentrated urine is a useful index of satisfactory resuscitation; less than 0.5 ml/kg/h indicates undertransfusion, while an output well over 1.0 ml/kg/h suggests overtransfusion. The hallmark of renal failure is severe tubular impairment that results in loss of concentrating power. If there is severe glomerular impairment as well, oliguria will be evident; otherwise the urine volume may be normal. Hence the importance of measuring urine concentration (osmolarity) or calculating the urine/blood urea ratio, which in normal health is usually 50 or more but in acute renal failure is less than 5. The triad of oliguria, loss of urine concentration and haem pigments in the urine is particularly ominous. A forced solute diuresis should be attempted (intravenous mannitol, 1 g/kg body weight as a 15 or 20% solution) whenever obvious ‘haemoglobinuria' occurs, but should not be repeated if a marked diuresis does not ensue. When examination of the urine output indicates serious renal impairment, the advice of a renal specialist should be obtained without delay. The enormous positive fluid balance typically present at 48 h postburn will, in the absence of effective renal function, threaten the patient's life by pulmonary oedema long before he is at risk of dying from uraemia.