Papillary muscles of Syrian hamsters with morphologically defined cardiomyopathy (CM), and of control hamsters (Co) have been studied electrophysiologically. Co and CM muscles differ significantly in the following electrophysiological parameters: resting potential (-79.8 +/- 2.9 mV in Co, -74.3 +/- 5.5 mV in CM), action potential duration at 25% repolarization (30.0 +/- 3.6 ms in Co, 25.1 +/- 2.7 ms in CM), and at 90% repolarization (120.0 +/- 21.9 ms in Co, 171.6 +/- 24.1 ms in CM), upstroke velocity of phase 0 depolarization (73.7 +/- 12.8 V/s in Co, 48.4 +/- 8.7 V/s in CM). When superfused with K+-free solutions Co muscles did not show any oscillatory after-potentials up to 15 mmol/l CaCl2. In contrast to this CM muscles respond to K+-free superfusion with oscillatory afterpotentials, transient spontaneous activity and dramatically reduced upstroke velocity. CM muscles respond much more sensitively to an intracellular Ca2+ overload than Co preparations.