The effect of water immersion to the neck on renal calcium and magnesium handling was studied in 11 nephrotic patients. There was an increase in the urinary excretion of both calcium and magnesium on immersion, and a return towards preimmersion control values in the hour following immersion. Clearances of calcium and magnesium, and fractional excretion of calcium and of magnesium all increased significantly during water immersion, and decreased in the postimmersion hour. However, magnesium excretion was 10–50 times greater than calcium excretion. Fractional excretion of sodium was highly significantly related to fractional excretion of calcium (p < 0.001) and magnesium (p < 0.001). The relationship between fractional excretion of phosphate and fractional excretion of calcium was statistically significant (p < 0.05), as was that between fractional excretion of phosphate and that of magnesium (p < 0.01). Magnesium and calcium fractional excretions were significantly correlated (p < 0.01). It was concluded that the increase in calcium and magnesium excretion on immersion is likely to be related to the sodium diuresis caused by central hypervolemia due to immersio