We studied the effects of insulin and of anti-insulin serum (AIS) on sodium excretion in isolated diabetic and normal, fed, rat kidneys perfused at a constant pressure of 100 mm Hg with Krebs-Ringer bicarbonate buffer containing 7.5% bovine serum albumin and 5 mM glucose and gassed with 95% O2 and 5% CO2. The addition of insulin produced antinatriuresis in kidneys from diabetic rats but not in those from normal rats. Moreover, before the addition of insulin, the baseline rate of sodium excretion was greater in the diabetic rat kidney than in the normal rat kidney, while no differences in glomerular filtration rate (GFR) were observed. The addition of AIS (enough to bind 1.4 mU of insulin per milliliter of perfusate) produced a brisk natriuresis in kidneys from normal rats, and this natriuresis could be reversed by the addition of saturating amounts of insulin. By contrast, the addition of normal guinea pig serum to the perfusate of normal kidneys or AIS to the perfusate of diabetic kidneys was unassociated with natriuresis. We conclude that (a) insulin promoted antinatriuresis in diabetic but not in normal rat kidneys, (b) AIS promoted natriuresis in normal kidneys, and (c) this natriuresis was a specific effect of AIS and was reversed by the addition of insulin. Our data support the view that insulin-induced antinatriuresis depends on endogenous levels of circulating insulin and on the availability of insulin-binding sites in the kidney.