Frog heart relaxation was analysed under voltage clamp conditions as the tension decay observed after membrane potential had been stepped back to its resting value. It appeared mostly exponential with a time constant of about 190 msec. Relaxation rate decreased with the external Na concentration. It fell to 1/10 in a Na-free solution. Relaxation rate decreased also following an increase in the intracellular Na content by application of veratrine. When switching from a high to a low Na-containing solution the relaxation rate reached rapidly a value transiently higher than the steady-state's (and vice versa). In Na-free solution, adrenaline accelerated tension decay, an effect not noticeable on frog heart in Ringer solution. In frog myocardium, part of the intracellular Ca concentration decrease which occurs during each cardiac cycle directly results from a calcium extrusion carried out by the Na-Ca exchange. Such a Ca decrease, besides internal Ca storage, should play a primary role in the relaxation of contractile tissues whose cells have a large surface/volume ratio.