1. A diuresis occurs within the first 36h of salt restriction. A decline in plasma arginine vasopressin concentration may contribute to both the diuresis and antinatriuresis. 2. We have studied six normal human subjects during 36h of dietary sodium restriction. In one study subjects received an intravenous infusion of D-glucose, and in the other an infusion of arginine vasopressin (6 fmol min-1 kg-1). 3. In the D-glucose phase plasma arginine vasopressin concentration fell (1.77 +/- 0.34 to 1.02 +/- 0.13 pg/ml), urine flow increased (67.9 +/- 11.5 to 89.8 +/- 17.1 ml/h), haemoconcentration occurred (packed cell volume 40.8 +/- 0.3 to 42.8 +/- 0.5%, protein concentration 71.6 +/- 0.5 to 74.5 +/- 0.6 g/l), plasma sodium concentration fell (140 +/- 0.2 to 138 +/- 0.2 mmol/l) and plasma renin activity increased (1600 +/- 153 to 3700 +/- 356 pg of angiotensin I h-1 ml-1). 4. In the arginine vasopressin phase plasma arginine vasopressin concentration remained constant (1.5 +/- 0.13 to 1.34 +/- 0.11 pg/ml), the diuresis was reversed (65.7 +/- 9.9 to 52.1 +/- 8.9 ml/h), plasma sodium concentration fell further (139.8 +/- 0.4 to 136.1 +/- 0.4 mmol/l), the rise in plasma renin activity was reduced (arginine vasopressin 2552 +/- 292; D-glucose, 3700 +/- 356 pg of angiotensin I h-1 ml-1) and creatinine clearance was lower in the last 12h of salt restriction (arginine vasopressin, 96.1 +/- 6.9; D-glucose 116.5 +/- 6.8 ml/min). Renal sodium excretion was unaffected by arginine vasopressin infusion. 5. We conclude that the fall in plasma arginine vasopressin concentration during dietary salt restriction, whilst not affecting renal sodium excretion, may be important in the regulation of plasma sodium concentration, plasma renin activity and glomerular filtration.