Using a sucrose gap, sinusoidal 50-cycle alternating current was passed through isolated papillary muscles of the rhesus monkey. Responses of the membrane potential accompanying AC polarization were recorded with intracellular microelektrodes. The following results were obtained: Upon AC polarization with just threshold intensity, several subthreshold oscillations of the membrane potential preceded the initiation of an all-or-none response (Gildemeister effect). Without any change in the effective strength of the polarizing current, the amplitudes of the local oscillations increased in the depolarizing direction up to threshold. Alternating current of 1.5 x rheobasic intensity gave rise to a rhythmic selective series of action potentials depending on refractory conditions. With a further increase of current strength, the rhythmic firing of action potentials ceased and the oscillation band of the membrane potential dislocated more and more to positive values, either forming a constant plateau of fluctuating rhythmically similar to electrical pacemaker activity. With isotonic replacement of all Na+-compounds of the Tyrode's solution by sucrose the membrane behaved like a rectifier with inward going rectification. Hence, under these conditions, AC polarization caused the membrane potential to oscillate asymmetrically about the resting potential, preferring the depolarizing half-periods. Time-dependent changes of the membrane conductance disappeared. In normal Thyrode's solution an increased mechanical tension was maintained throughout AC polarization if the average potential level of the oscillation band became more positive than about−30 mV. Possible ionic current mechanisms concerning the movement of Na+-or K+-ions, respectively, which may account for some of the observed effects were analysed by means of an electronic computer model of the cardiac fibre membrane.