In 11 patients suffering from nephrotic syndrome, renal phosphate handling was studied in water immersion up to the neck. There was a rapid onset of phosphate diuresis which continued to increase throughout water immersion; and an increase in fractional excretion of phosphate (FE<sub>PO4</sub>) continuing throughout immersion and decreasing in the hour after immersion. TmPO<sub>4</sub>/GFR did not change on immersion. TmPO<sub>4</sub>/Inulin clearance gave higher figures than TmPO<sub>4</sub>creatinine clearance due to secretion of creatinine in 9 of 11 patients. There was a transitory increase in PTH during immersion, but mean immersion plasma levels of PTH did not change when compared to preimmersion control values. FE<sub>PO4</sub> was more frequently significantly correlated with FE<sub>C1</sub> than to FE<sub>Na</sub> in nephrotics.