Despite the effects of insulinopenia in type 1 diabetes and evidence that insulin stimulates multiple renal sodium transporters, it is not known whether normal variation in plasma insulin regulates sodium homeostasis physiologically. This study tested whether the normal postprandial increase in plasma insulin significantly attenuates renal sodium and volume losses. Rats were instrumented with chronic artery and vein catheters, housed in metabolic cages, and connected to hydraulic swivels. Measurements of urine volume and sodium excretion (UNaV) over 24 hrs and the 4-hr postprandial period were made in Control (C) rats and Insulin-Clamped (IC) rats in which the postprandial increase in insulin was prevented. 24-hr urine volume (36±3 vs. 15±2 ml/day) and UNaV (3.0±0.2 vs. 2.5±0.2 mmol/day) were greater in the IC compared to C rats, respectively. Four hrs after rats were given a gel meal, blood glucose and urine volume were greater in IC rats, but UNaV decreased. To simulate a meal while controlling blood glucose, C and IC rats received a glucose bolus that yielded peak increases in blood glucose that were not different between groups. Urine volume (9.7±0.7 vs. 6.0±0.8 ml/4-hrs) and UNaV (0.50±0.08 vs. 0.20±0.06 mmol/4-hrs) were greater in the IC vs. C rats, respectively, over the 4 hr test. These data demonstrate that the normal increase in circulating insulin in response to hyperglycemia may be required to prevent excessive renal sodium and volume losses, and suggest that insulin may be a physiologic regulator of sodium balance.