In the present study we have analyzed a likely biochemical mechanism underlying the Ca++-sensitizing action of MCI-154 (6-[4-(4'-pyridyl)aminophenyl)-4,5-dihydro-3(2H)-pyridazinone hydrochloride), a novel cardiotonic agent, on the contractile protein system. MCI-154 (10(-7) to 10(-4) M) enhanced the tension development induced by -log molar-free Ca++ concentration (pCa) 5.8 in chemically skinned fiber from the canine right ventricular muscle in a concentration-dependent manner. At pCa 7.0, MCI-154 (10(-7) to 10(-4) M) markedly increased adenosine triphosphatase (ATPase) activities of canine myofibrils and reconstituted actomyosin. In myofibrils and reconstituted actomyosin, MCI-154 (10(-7) to 10(-4) M) caused a parallel shift of the pCa-ATPase activity relation curve to the left without affecting the maximum activity, suggesting an increase in Ca++ sensitivity. MCI-154 (10(-8) to 10(-4) M) had little effect on actin-activated, Mg++, Ca++ and (K+, EDTA)-ATPase activities of myosin. Ca++ binding to cardiac myofibrils or purified cardiac troponin was increased by 10(-4) M MCI-154. These results suggest that MCI-154 enhances Ca++ binding to cardiac troponin C to elevate the Ca++ sensitivity of myofilaments and thus may cause a positive inotropic action in cardiac muscle. MCI-154 may provide a valuable tool for studying the molecular mechanism by which Ca++ regulates the contractile system.