Action potentials were recorded from different regions of the guinea pig ventricle to characterize regional differences in waveform configuration, and to acquire insight into the generation of the T-wave of the electrocardiogram. Isolated tissue preparations were driven at 1 Hz, and microelectrodes were used to map accessible surface regions of the epicardium, endocardium, and septum. There were minimal differences in regional resting potentials (mean -87 mV) and amplitudes (mean 122 mV), but Vmax in the epicardium (mean 110 V/sec) was much smaller than elsewhere (mean 247 V/sec). The action potential duration at the -80 mV repolarization level was longest in the papillary muscles (mean 154 msec), shortest in the septum (mean 126 msec), and generally 10-15 msec longer at the base than at the apex. The characteristics of intramural action potentials were inferred from measurements on enzymatically isolated myocytes, the rationale being that most dissociated myocytes originated from intramural cell layers. The action potentials in about 40% of the myocytes had durations similar to those recorded from the tissue surface (110-170 msec), and the remainder ranged from 170-290 msec long. The existence of longer-than-surface action potentials in the ventricle was also inferred from the body surface electrocardiogram and from bipolar electrograms of isolated left ventricles. In both cases, the Q-T intervals could be accounted for only by action potentials longer than those recorded from the ventricular surface.