We wished to elucidate the effects of the calcium-sensitizing positive inotropic agent MCI-154 and its combined use with an angiotensin-converting enzyme (ACE) inhibitor enalapril on postischemic contractile dysfunction. Anesthetized dogs underwent a 30-min occlusion of the left anterior descending coronary artery (LAD) followed by 2 h of reperfusion. Regional myocardial segment shortening in the ischemic LAD area was assessed by sonomicrometry. Myocardial segment shortening decreased in response to the LAD occlusion and remained decreased during 2-h reperfusion. The intravenous infusion of MCI-154 (0.1 or 0.3 micrograms/kg/min) initiated 10 min after occlusion and throughout reperfusion significantly improved the recovery of segment shortening. The alleviation of the postischemic contractile dysfunction by MCI-154 was augmented when the animals were treated with a bous injection of enalapril (0.3 mg/kg) 15 min before ischemia followed by an infusion of the drug (0.003 mg/kg/min). The pretreatment with enalapril alone (0.3 mg/kg plus 0.003 mg/kg/min or 1 mg/kg plus 0.01 mg/kg/min) did not alleviate the postichemic dysfunction, however, although it decreased systemic blood pressure (BP). Ischemic bed size, myocardial necrosis (by triphenyltetrazolium chloride staining), and collateral blood flow (by colored microspheres) were similar in all experimental groups. These results indicate that MCI-154 improves the postischemic contractile function of dog heart, whereas enalapril fails to improve it. ACE inhibitors may also augment the efficacy of cardiotonics on postischemic dysfunction.