The contractile apparatus of mechanically skinned muscle fibres of frog can be reversibly activated by Ba2+ and Cd2+. The maximum force induced by both Ba2+ and Cd2+ is the same as that induced by Ca2+ and Sr2+. The ionic concentrations of the divalent cations required to induce 50% of the maximum activated force at 1 mM-Mg2+, pH 7.10, 22 degrees C and 250 mM ionic strength are about 8 X 10(-7) M for Ca2+, 5 X 10(-6) M for Cd2+, 2.6 X 10(-5) M for Sr2+ and 7 X 10(-4) M for Ba2+. Exposure of the skinned fibre to relatively low Ni2+ concentrations (between 10(-6) and 10(-5) M) resulted in a transient force response accompanied by an irreversible change in the ability of the preparation to develop force. The Ba2+- and Cd2+-activation curves are considerably flatter than the corresponding curves for Ca2+ and Sr2+. An increase in Mg2+ concentration from 1 to 3 mM decreased the sensitivity of the contractile apparatus to Ba2+ by a factor of about 1.5 without affecting the maximum force response. The Ca2+-activation curve was modified in the presence of subthreshold concentrations of Ba2+ and the results indicate that Ba2+ could have both an activating and an inhibitory action on the Ca2+-activated force. A kinetic model which can quantitatively explain the results for activation of contraction by Ba2+ and Ca2+, is described.