Experiments were performed in human working myocardium to investigate the relationship of intracellular calcium handling and availability to alterations in the strength of contraction produced by changes in stimulation rate and pattern. Both control and myopathic muscles exhibited potentiation of peak isometric force during the postextrasystolic contraction which was associated with an increase in the peak intracellular calcium transient. Frequency-related force potentiation was attenuated in myopathic muscles compared to controls. This occurred despite an increase in resting intracellular calcium and in the peak amplitude of the calcium transient as detected with aequorin. Therefore, abnormalities in contractile function of myopathic muscles during frequency-related force potentiation are not due to decreased availability of intracellular calcium, but more likely reflect differences in myofibrillar calcium responsiveness. Sarcolemmal calcium influx may also contribute to frequency-related changes in contractile force in myopathic muscles as suggested by a decrease in action potential duration with increasing stimulation frequency which is associated with fluctuations in peak calcium transient amplitude.