The cardiovascular effects of intracerebroventricular (i.c.v.) and intravenous (i.v.) injection of a selective vasopressin vasopressor antagonist, [1-(β-mercapto-β, β-cyclopentamethylenepropionic acid-2-(0-methyl)tyrosine]arginine vasopressin (TMe-AVP) were examined in conscious rats under basal conditions and following 48 h of water deprivation. Pressor responses to i.v. vasopressin (50 ng/kg) were not blunted by i.c.v. treatment with either vehicle or 0.5 µg/kg TMe-AVP. A dose of 5.0 µg/kg TMe-AVP (i.c.v.) did reduce the pressor response to vasopressin, indicating peripheral leakage of the antagonist. Water deprivation for 48 h increased plasma vasopressin concentrations from 0.7 ± 0.1 to 2.8 ± 0.1 µU/ml and increased blood pressure from 112 ± 2 to 123 ± 1 mm Hg. No effect of the vasopressin antagonist on blood pressure could be detected following either i.c.v. (0.5 µg/kg) or i.v. (5.0 µg/kg) treatment in water-deprived animals. However, a significant increase in heart rate was observed in water-deprived rats following i.v. injection of 5.0 µg/kg of TMe-AVP. Central vasopressin vasopressor receptor blockade appears to exert little effect on blood pressure either under basal conditions or during water deprivation. The data further delineate the relationship of plasma vasopressin concentrations to cardiovascular homeostasis.