Dibutyryl cyclic-AMP was administered to a group of anesthetized dogs at the doses of 5, 10, 20 and 40 mg/kg, and changes in stroke volume, cardiac output and myocardial contractility were observed. Also, effects of this substance on contractile performance of isolated cardiac muscle were investigated in another group of dogs. Results were as follows: 1) Dibutyryl cyclic-AMP increased heart rate, stroke volume and cardiac output in a dose-related manner in the range from 5 to 40 mg/kg. 2) As indices of myocardial contractility, Vmax and maximum dF/dt were examined. These also showed a dose-dependent increase in response to dibutyryl cyclic-AMP. In isolated dog hearts, dibutyryl cyclic-AMP raised the maximum velocity of shortening (V'max) dose-dependently. 3) Total peripheral resistance declined in a dose-related way. On the other hand, mean arterial pressure and CVP dropped slightly without showing dose-dependence. These results show that dibutyryl cyclic-AMP produces positive inotropic and chronotropic effects and cause dilatation of the peripheral vessels in dose-dependent manners.