1. The effects of the physiological levels of corticotropin-releasing factor (CRF) on isolated guinea-pig ventricular myocardium were studied using a force transducer and standard microelectrode techniques. 2. CRF increased the contractile force of muscles concentration-dependently in normal Tyrode and a high-K+ (27 mM) solution. The positive inotropic effect of CRF was associated with a significant enhancement of the slow action potentials of partially depolarized muscles in high-K+ solution. CRF potentiated the effect of increasing Ca2+ concentration of Tyrode solution. 3. The inotropic effect of CRF was reduced in the presence of diltiazem, and suppressed by phentolamine, metoclopramide, and cimetidine, but was not affected by propranolol and cold condition. 4. It is suggested that an increase in the slow inward Ca2+ current induced by CRF plays an important role in its positive inotropic effect and that its effect differs from that of cardiotonic steroids.