Pulmonary and systemic hemodynamic effects of delta 9-tetrahydrocannabinol (delta 9-THC) in conscious dogs and in those anesthetized with morphine (3 mg/kg, i.m.) plus alpha-chloralose (100 mg/kg i.v.) were evaluated in this study. A decrease in the heart rate, cardiac output (PBF) and a concomitant increase in the pulmonary arterial pressure (PAP), pulmonary vascular resistance (PVR) and right ventricular stroke work (RVSW) observed in conscious animals following the administration of delta 9-THC were qualitatively similar to the effects reported in dogs anesthetized with sodium pentobarbital; however, unlike in the pentobarbital group, hypotensive effects to THC were not evident in the consious animals. In contrast, the effect of delta 9-THC in morphine--chloralose dogs were different; in this group delta 9-THC administration resulted in increases in the heart rate the PBF, and significant reductions in PAR, PVR and RVSW. Further, a decrease in the arterial blood pressure noted following THC administration was closely associated with a reduction in the total peripheral resistance in the morphine--chloralose group. The results of this study indicated that the pulmonary effects of THC in dogs may be related to its actions on the heart rate and differ qualitatively as well as quantitatively depending on the anesthetic used.