BACKGROUND A study of the effects of hyper -and hypotonic media on resting tension were examined in ventricular muscle of the bullfrog was undertaken because only few studies on resting tension from the viewpoint of E-C coupling in the heart muscle are available. METHODS Thin strips obtained from the bullfrog ventricle were perfused in the sucrose-gapped chamber, which allowed the effective stimulation and the simultaneous measurement of the contractile tension and the intracellular membrane potential. CONCLUSIONS Hypertonic solutions up to 3.0 T, prepared by adding NaCl or sucrose, induced a triphasic response, namely an initial rapid decline, a secondary slow development of contracture and a final transient off-contraction. Both the second and the final responses were strongly sensitive to the calcium concentration of the bathing medium and were completely abolished by depleting external calcium ions. The initial decreasing phase was not sensitive to external calcium but was progressively augmented by increasing the initial muscle length, whereas the dehydration process, estimated by measuring the muscle weight, was unaffected by muscle stretch. The changes in the membrane potential were not essential for these responses. It was concluded that the initial tension decline results from the passive process due to volume change while the sustained contracture as well as the transient off-contraction are produced by some active process involved in E-C coupling. Hypotonic solutions induced a monophasic contracture which is insensitive both to the external calcium concentration and muscle stretch. The possible mechanism for the genesis of the response was discussed.