Ten anephric patients were studied before and during hemodialysis. The extracorporeal circuit was primed with 5% albumin in 0.9% sodium chloride. Ultrafiltration volume removed by the hemodialyzer was replaced continuously. Modifications of a standard chronic renal failure dialysate were used to minimize changes in plasma urea while varying plasma sodium and calcium in opposite directions. Plasma ionized calcium concentrations in two patients confirmed other studies demonstrating a correlation between plasma total calcium and ionized calcium under these conditions. Plasma ADH determined by bioassay did not correlate with plasma osmolality, plasma sodium concentration, plasma potassium concentration, blood pressure, or pulse rate. The change in plasma ADH during dialysis was significantly correlated only with the change in plasma calcium (r = 0.47, p < 0.05). The data support the hypothesis that plasma calcium plays a role in the regulation of ADH release in man, independent of the renin-aldosterone system.