Chronic administration of delta9-tetrahydrocannabinol (delta9-THC) 1mg/1kg, s.c., twice a day for 7 days failed to produce any major changes in the pulmonary or systemic hemodynamics and in the myocardial contractility of the mongrel dogs. Chronic delta9-THC treatment did not result in any significant alterations either in the transmission in the lumbar sympathetic chain or in the activity of autonomic receptors within the cardiovascular system. Further systemic pressor responses to bilateral carotid occlusion and reflexly mediated alterations in the perfused hindlimb vascular resistance were essentially identical in both the treated and placebo groups. Pressure-flow curves obtained from the perfused hindlimb preparations suggested that in contrast to the acute studies, neurogenic tone to the hindlimb vasculature was unaltered by chronic delta9-THC and intrinsic tone of the vascular smooth muscle was significantly depressed in the treated animals; in addition it appears that the tone due to circulating catecholamines was significantly enhanced in treated animals. The results of this investigation demonstrated that several acute cardiovascular and autonomic effects of delta9-THC may not be evident following chronic THC-treatment, however, prolonged use may demonstrate certain new pharmacological properties and/or mechanism of actions which cannot readily be seen in the acute studies.