Cardiovascular and autonomic functions of the dogs treated with delta 9-Tetrahydrocannabinol (delta 9-THC) 2 mg/kg/day for 35 days, were evaluated under pentobarbital anesthesia. Treated animals required significantly less pentobarbital in comparison with that of the placebo group. However, the interaction between delta 9-THC and pentobarbital at central vagal structures noted in the 7-day chronic study was not evident in the present study. Similarly, vagolytic effects of THC reported in acute studies could not be demonstrated following 35-day treatment. Reflex bradycardia responses to intravenous norepinephrine and phenylephrine were potentiated in the treated animal. These actions were similar to that of clonidine; however, unlike clonidine, this potentiation was due to enhanced withdrawal of sympathetic tone. While there was a significant reduction in the resistances of mesenteric and femoral vascular beds, the overall pulmonary and systemic hemodynamics of the treated animals remained unchanged. An analysis of the ventricular function curves indicated the prolonged administration of delta 9-THC to the mongrel dogs for 35 days failed to produce any adverse alterations in the myocardial contractility and function.