1. A method is described by which the solutions bathing single auricular trabeculae, isolated from the heart of the frog, can be rapidly altered while the tension generated and the membrane potential can be measured simultaneously.2. Changes of the [Ca](o) result in changes of the twitch strength similar to that reported for frog ventricle.3. At [Ca](o) of less than 1 mM, the isometric contracture tension generated during application of K-rich solutions, and the maximum rate of tension development, are proportional to [Ca](o) (3).4. This relationship is not the consequence of (a) the hypertonicity of the K-rich solutions, (b) the dependence of the membrane potential on [Ca](o), or (c) the facilitation due to a twitch response at the initiation of the contracture.5. Reduction of the [Na](o) increases the strength of the high-K contractures according to the ratio of [Ca](o)/[Na](o) (2); Na ions in the bathing medium are shown to competitively inhibit the potentiating action of Ca ions on the force generated during contractures.6. An equation is derived which assumes that three Ca compounds act co-operatively at some stage in the process of excitation-contraction coupling.7. Two hypotheses are discussed. The first proposes that the sarcoplasmic [Ca] established during depolarization of the muscle membrane depends upon [Ca](o) (3), and tension generated by the contractile elements on a first order reaction with ionic Ca. The second suggests that if the sarcoplasm [Ca] established during excitation is proportional to [Ca](o), then three Ca ions are required to activate the contractile response at the unit level.