A possibility of interaction between effects of insulin and vasopressin (ADH) on renal handling of water and electrolytes was examined in continuously hydrated rats whose hypothalamo-hypophysial antidiuretic system was blocked with ethanol. Urine (Vu), its osmolality (Uosm) as well as sodium (UNaV) and potassium (UKV) excretion were compared before and after: 1) single intravenous (i.v.) injection of insulin (10, 20 and 40 mU), 2) i.v. infusion of ADH (5 microunits/min) for 20 min, and 3) injection of insulin followed by infusion of ADH. Administration of insulin without ADH caused a significant increase in Vu and a decrease in Uosm. A significant increase in UNaV was found after 10 mU of insulin. Infusion of ADH caused a significant decrease in Vu, elevation of Uosm and small but significant increase in UNaV. Simultaneous administration of insulin and vasopressin did not cause any significant changes of Vu and Uosm. Administration of 10 mU of insulin in association with ADH elicited a highly significant prolonged increase in UNaV which was higher than that observed when each hormone was applied separately but lower than the sum of separate responses to ADH and insulin. The results indicate that insulin and ADH exert opposite effects on urine flow and osmolality and that the low doses of insulin and ADH act synergistically in eliciting natriuresis.