1. Using conventional microelectrode techniques for the intracellular recordings of the membrane potential, the effects of labetalol were studied on cardiac Purkinje, atrial and ventricular muscle fibres of the dog. 2. Labetalol (1-10 microM) reduced, in a concentration-dependent manner, the action potential amplitude (APA) and the maximum rate of rise of the action potential (Vmax) in Purkinje fibres. 3. The action potential duration (APD) was decreased in Purkinje fibres but significantly increased in ventricular fibres after small concentrations of labetalol (1-3 microM). The atrial fibres were not very sensitive to labetalol. 4. Depolarization of the cardiac Purkinje fibres by increasing the external potassium concentration (8-12 mM), potentiated the labetalol effects on APA and Vmax but blocked its effects on the APD. 5. The effects of labetalol on Vmax of Purkinje fibres were dependent on the frequency of stimulation. 6. The ratio of the effective refractory period to the APD was increased both in normally polarized and depolarized Purkinje fibres after treatment with labetalol (10 microM). 7. Labetalol (10 microM) shifted the membrane responsiveness curve of Purkinje fibres by about 10 mV in the hyperpolarizing direction. 8. The slow response obtained in K-depolarized, Ba-treated Purkinje fibres was not significantly affected by labetalol (10-100 microM). 9. It is suggested that labetalol can exert Class I and Class III antiarrhythmic actions in cardiac muscle of the dog in vitro.