Experiments were designed to investigate the importance of the endothelium in the relaxation of isolated rat aorta caused by a beta adrenoceptor agonist. Mechanical removal of the endothelium attenuated the relaxation induced by isoproterenol (ISO) and did not affect the relaxation produced by forskolin and by sodium nitroprusside. High concentrations of ISO produced an increase in the resting tension of aortic strips with and without endothelium in a concentration-dependent manner. Mechanical removal of the endothelium or treatment with methylene blue enhanced the maximal contraction induced by ISO. Phentolamine antagonized the contractile responses induced by ISO. In the case of streptozotocin-induced diabetic rats, both aortic strips with and without endothelium generated concentration-response curves for ISO-induced relaxation that were shifted to the right. The relaxant responses to forskolin and sodium nitroprusside were not significantly different between vessels from diabetic and age-matched control rats. In both aortic strips with and without endothelium, the maximal contraction in response to high concentrations of ISO was significantly enhanced in strips from diabetic rats. These results suggest that ISO-induced relaxation of aortic strips with endothelium is mediated by beta adrenoceptors on both the endothelium and the smooth muscle, and high concentrations of ISO produce an increase in the resting tension through alpha adrenoceptors. It is further suggested that the decreased relaxant response of the aorta to ISO in diabetes may be due to decreased density or affinity of beta adrenoceptors on the smooth muscle.