The food and fluid intake, the fecal weight and weight of urine voided, urinary and plasma osmolality and neurohypophysial content of vasopressin and oxytocin were measured in groups of rats injected with oil and vasopressin (0.5 IU Pitressin tannate in oil daily, i.m.) before, during and after substitution of a 2% solution of NaCl for drinking water for 3 days. Before 2% NaCl was substituted for the drinking water, vasopressin treatment significantly decreased food and water intake (p < 0.05) and daily weight gain (p < 0.01), but no significant effect on plasma osmolality or on neurohypophysial content of vasopressin and oxytocin could be demonstrated. Vasopressin treatment did not significantly reduce the intake of the 2% NaCl solution when this was substituted for drinking water but did reduce the resulting neurohypophysial depletion of vasopressin (p < 0.01). Furthermore, on the first day of NaCl drinking, the neurohypophysial content of vasopressin in vasopressin-treated rats was increased above the control value (p < 0.05). These results suggest the existence of a negative feedback of vasopressin on its own release.