Recent evidence indicates that the plasma vasopressin concentration is higher in males than in females and that this may be due to sexually dimorphic effects of the gonadal steroids. However, whether this difference in plasma vasopressin levels reflects differences in secretion or metabolism of the hormone could not be determined from the available data. Therefore, we measured the MCR of vasopressin in conscious male and female rats over a broad range of plasma concentrations of the hormone. No differences were observed in MCR either within a sex or between the sexes over the ranges of plasma concentrations of vasopressin tested. This indicates that the differences in basal plasma vasopressin concentration that were previously reported between male and female rats reflect differences in secretion. In addition, the pressor responses to infused vasopressin were 2 to 3 times greater (P less than 0.01) in male than in female rats, even though the plasma vasopressin concentrations achieved in the two groups were identical. Although the basal heart rate was higher (P less than 0.01) in female than in male rats, the decreases in heart rate observed in response to the vasopressin infusions were similar between the sexes. The mechanisms responsible for the differences in basal vasopressin secretion and pressor responsiveness between males and females are uncertain, but could involve actions of the gonadal steroids on the central nervous system and the peripheral vasculature.