The cardiovascular effects of arginine vasopressin (AVP) administered into a lateral cerebral ventricle or into the cisterna magna were investigated in conscious Long Evans (control) rats and AVP-deficient Brattleboro rats. The effects of subpressor intracerebroventricular and intracisternal doses of AVP on cardiac baroreflex sensitivities were also determined. Intracerebroventricular and intracisternal AVP increased blood pressure of both strains of rat in a dose-dependent manner. The maximum pressor response produced by intracerebroventricular AVP in Long Evans rats was 13 ± 2/13 ± 1 mm Hg (systolic/diastolic,n=6) after 100 ng AVP. The pressor response to the highest intracerebroventricular dose of AVP tested in Brattleboro rats (30 ng) was 46 ± 13/21 ± 6 mm Hg (n=6). Intracerebroventricular AVP caused a tachycardia in Brattleboro rats but had no effect on heart rate of Long Evans rats. At doses greater than 1 ng, the increases in blood pressure produced by intracisternal AVP in both groups of rats were significantly greater than the increases produced by the same doses given intracerebroventricularly. Heart rate fell in a dose-dependent manner after intracisternal AVP in Long Evans rats but not in Brattleboro rats. Cardiac baroreflex sensitivities of Brattleboro rats were not significantly different from those of Long Evans rats and were not modified by intracerebroventricular (0.3 ng) or intracisternal (0.1 ng) AVP. In Long Evans rats, intracisternal AVP (0.3 ng) increased cardiac baroreflex responses to both increases and decreases in pressure. Intracerebroventricular AVP (0.3 ng) increased the sensitivity of the reflex in response to an elevation but not to a reduction in blood pressure. These results suggest that Brattleboro rats are more sensitive than Long Evans rats to the central pressor action of AVP and indicate that AVP can modify the baroreflex of Long Evans rats at a site(s) within the brain. Brattleboro rats may maintain normal baroreflex function through mechanisms that do not use, and are not modified by, central AVP.