We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na⁺–Ca²⁺ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca²⁺ transient but increased intracellular pH and Ca²⁺ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na⁺–H⁺ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca²⁺ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET_A receptors, activation of the Na⁺–H⁺ exchanger and an increase in intracellular pH and Ca²⁺ sensitivity and that the negative inotropy is mediated by ET_B receptors, activation of the Na⁺–Ca²⁺ exchanger and decrease in Ca²⁺ transient amplitude.