This study was performed to determine if the intracellular electrical activity of adult cardiac papillary muscle from a strain of Syrian hamster (B10 14.6) with a genetically determined cardiomyopathy was different from that of a control strain (B10 RB). Muscles were driven at rates from 1 Hz through the physiological range. Resting potentials of muscle from the two strains were not significantly different, except in increased [C2+]0, where the control was slightly hyperpolarised. Action potential overshoot was significantly greater in the myopathic at all frequencies, while the upstroke velocity was not significantly different, except at 8 Hz where the myopathic value was depressed. Action potential durations at 20, 50 and 95% repolarisation were significantly greater in the myopathic, at all rates and with increased [C2+]0. Response of the muscles to verapamil and D600 were complex, with a decrease seen in duration at 20% repolarisation and an increase in 50% duration at 1 Hz rate. No strain-related difference in sensitivity to the drugs was seen. The results support the hypothesis that a sarcolemmal defect is intimately related to the genesis of the cardiomyopathy.