Isolated perfused guinea pig hearts (Langendorff preparation) were used to study the influence of direct current (DC) on the shape of the electrogram and to determine the thresholds for extrasystoles and for fibrillation. DC impulse durations ranged between 30 ms and 5 s. During DC flow through the heart, marked deformations of the electrogram occur mainly concerning the repolarization phase and increasing with growing current strength. At an intensity of twice the diastolic threshold for stimulation, constant DC induces extrasystoles, which are suggested to result from an enhancement of the intrinsic automatic activity by cathodal polarization. Compared with the diastolic threshold for stimulation, the threshold for fibrillation is higher by a factor of about 15. The curve relating the threshold for fibrillation to the impulse duration shows a marked decline between 30 ms and 60 ms, followed by a slight increase and adjustment to a constant level beyond about 80 ms. In the used setup, the threshold for fibrillation is independent on the reversal of polarity of the electrodes. Long-lasting DC pulses are able to induce fibrillation even if they are switched on in the absolute refractory period of a normal beat. Application of 50-Hz alternating current revealed a higher threshold for stimulation and a three times lower fibrillation threshold compared with the effects of DC in the same heart.