1. Eleven nephrotic patients were immersed up to the neck in 1.3 m of water at 34°C for 4 h. 2. A diuresis began, within the first hour of, and continued throughout, immersion. The urine osmolality fell significantly and decreased to hypotonic levels in the first hour of immersion and increased gradually during the remainder of immersion and postimmersion. 3. Throughout immersion there was a steady increase in urinary sodium and chloride excretion and a sharp fall in the postimmersion hour. 4. Overall fractional excretion of sodium and of chloride increased steadily throughout immersion and fell during the post-immersion hour. 5. There was an increase in urinary potassium excretion on immersion but no change in fractional excretion of potassium. There was an initial increase in distally reabsorbed sodium/chloride on immersion, as expressed by positive values of C H2O in the first hour of immersion. 6. Plasma aldosterone levels were initially elevated in only two out of nine patients. There was a significant fall in plasma aldosterone levels on immersion, but there was no correlation between sodium/chloride handling and aldosterone levels on immersion.