The diuretic and natriuretic response to water immersion, which is known to increase effective central blood volume, was studied in eight edematous children with nephrotic syndrome. The rise in central blood volume was indicated by a decrease in hematocrit from preimmersion median of 40.2% to 38.6% during water immersion (P less than 0.05, Friedman test). Similarly, serum protein concentration fell from 36.7 gm/L to 33.5 gm/L (P less than 0.05). Water immersion induced diuresis from a preimmersion median of 0.33 ml/min/1.73 m2 to 1.52 ml/min/1.73 m2 (P less than 0.05). Osmolar clearance rose, as did sodium and potassium excretion. Urine osmolality fell during water immersion (P less than 0.05). Serum sodium concentration and plasma osmolality did not change. Plasma arginine vasopressin values fell from 11.1 pg/ml to 3.0 pg/ml (P less than 0.05), as did renin activity (8.5 ng to 5.2 ng angiotensin l/ml/hr, P less than 0.01), aldosterone (18.0 ng/dl to 10.1 ng/dl), and norepinephrine (344 pg/ml to 213 pg/ml, P less than 0.05). Water immersion produces a potent natriuretic and diuretic response in children with nephrotic syndrome.