Isometric twitches and intra- and extracellular electrical activity were recorded at various driving rates from lizard ventricles, either in the normal inotropic state, or in a hypodynamic state induced by perfusing at a high flow rate for a long time. Electron micrographs were obtained from hearts fixed immediately after dissection and from preparations perfused in vitro for various periods. It was found that the peak of the steady-state strength-interval relationship shifts towards higher stimulation rates with the development of hypodynamia. Such a change is similar to that induced in normal preparations by perfusing with calcium-poor solutions. The normal strength-interval relationship can be restored in hypodynamic preparations by perfusion with a calcium-rich solution. Also, in the hypodynamic state, the action potential duration is increased and (+ dP/dt)max is decreased, while the shape of the staircases at various stimulation rates is modified. These changes occur in the presence of well-preserved ultrastructural features of the preparation. The results suggest that the twitch tension results from the contribution of two Ca ions fluxes: an early one from a cellular store, and a late one related to the action potential duration. The ultrastructural findings are consistent with the hypothesis that a store, from which a rapid release of calcium occurs, exists in the lizard ventricle. A hypodynamic state would be caused by a reduced calcium affinity of such store, and by a decreased Ca++ influx during the action potential plateau.