When the concentration of external Ca2+ was reduced for 30 sec in a single twitch muscle fiber of a frog, the peak tension of the initial component of biphasic 80 mM K+ contractures was potentiated, whereas that of the secondary component was markedly inhibited, despite the fact that in the early stage it was potentiated usually in case of contractures in 60 and 40 nM K+ but rarely in 80 mM K+. These changes were not observed, however, in the presence of 0.5-1 nM Mn2+, i.e., Mn2+ having been substituted for Ca2+. The foregoing result and the authors' previously reported data indicated the following. First, the concentrations of divalent cations having an equal effect in bringing about the peak tensions of both components are 3 mM Mg2+, 0.7 mM Mn2+, 0.5 mM Ni2+, and 1.8 mM Ca2+. Secondly, this sequence constitutes their increasing order of effectiveness on the time course of the secondary component. Meanwhile, a similar order was found to exist in another experiment concerning the effectiveness in inhibiting the inactivation of K contractures facilitated by lowering the concentration of external Ca2+. All these findings indicated that the divalent cations act on the activation processes of both components in a stabilizing manner, differing from the way in which they act on the inactivation process of the secondary component. The mechanisms in which the peak tensions of the initial and secondary components are inhibited in a low Ca2+ solution and the divalent cations act on both components are discussed. Finally, another experiment made in the absence and presence of Ca2+ revealed that the effect of high concentrations of Mn2+ in the initial component is different from that on the secondary component.