In the anesthetized open-chest guinea pig, ibopamine (10-300 micrograms/kg, i.v.), epinine (10-100 micrograms/kg, i.v.) and dopamine (10-300 micrograms/kg, i.v.) produced dose-related increases in heart rate (prevented by 20 micrograms/kg pindolol, i.v.), left ventricular dP/dt max and aortic flow. Ibopamine produced pressor effects (prevented by 0.5 mg/kg phentolamine, i.v.), while dopamine produced a slight depressor effect. A biphasic response (the pressor phase followed by a depressor) was observed after epinine, although the depressor phase was not significant. Calculated total peripheral resistance (TPR) tended to be increased after ibopamine and epinine (initial phase), while it was decreased after dopamine. Pindolol potentiated the increase in TPR produced by ibopamine and epinine, while the increase in TPR was converted to the decrease after phentolamine. Decreases in TPR produced by epinine and the highest dose of dopamine were inhibited by pindolol. In the isolated guinea pig atria, ibopamine (10(-6)-10(-4) M) increased the atrial rate and the developed tension in a concentration-related manner. The positive chronotropic and inotropic effects of ibopamine were of the same order as those of epinine.