To examine the osmotic control of vasopressin release, hypertonic saline was infused in conscious unrestrained male rats and female rats in each phase of the estrous cycle. The progressive increase in plasma osmolality was accompanied by a progressive increase in the plasma vasopressin concentration, but the magnitude of the former was smaller in metestrus than in other phases of the cycle and in males (P less than 0.01). The osmotic threshold for vasopressin release was higher in males than in females in each phase of the estrous cycle (P less than 0.05 to P less than 0.01), but the sensitivity of the osmotic control of vasopressin release was similar in male and female rats. Although the pressor response to hypertonic saline was greater in estrous females than in the other females (P less than 0.05 to P less than 0.01), the reduction in blood pressure, after a V1 antagonist 30 min after starting the hypertonic saline infusion, was similar in males and females. Thus there are gender-related differences in the osmotic control of vasopressin release and, in females, cycle-dependent differences in the cardiovascular responses to hypertonic saline.