Active sliding between doublet microtubules of sea urchin sperm axonemes that were demembranated with Triton X-100 in the presence or absence of calcium was induced with ATP and elastase at various concentrations of Ca2+ to examine the effects of Ca2+ on the direction of the power stroke of the dynein arms. Dark-field light microscopy of microtubule sliding revealed that the sliding from the axonemes demembranated with Triton and millimolar calcium and disintegrated with ATP and elastase showed various patterns of sliding disintegration, including loops of doublet microtubules formed near the head or the basal body. These loops were often thicker than the remaining axonemal bundle. In contrast, only thinner loops were found from the axonemes demembranated with Triton in the absence of calcium and disintegrated with ATP and elastase at high Ca2+ concentrations. Electron microscopic examination of the direction of microtubule sliding showed that the doublet microtubules in the axonemes demembranated in the presence of millimolar calcium moved toward the base of the axonemes by the dynein arms on the adjacent doublet microtubule as well as by their own dynein arms. Doublet microtubules in the axonemes demembranated in the absence of calcium moved toward the base of the axonemes only by their own dynein arms. Similar observations have been obtained from the axonemes from which the outer dynein arms were selectively extracted. From these observations, we can conclude that the dynein arms generate force in both directions and this feature of the dynein arms arises from at least the inner dynein arms.