Sotalol is not only a beta blocker but a class III antiarrhythmic drug. Its possible antifibrillatory activity was therefore investigated in both the ventricles and atria of dog heart in situ, since vulnerability to fibrillation is not the same in both these parts of the myocardium. Fibrillation threshold was measured concurrently with the duration and amplitude of monophasic action potential, the effective refractory period, the conduction time in the contractile fibres, and after fibrillation had been triggered the fibrillation rate. Variables were measured at 5 and 10 min after sotalol had been given intravenously in closed chest dogs in three doses (1, 1, and 2 mg X kg-1) at 15 min interval. Sotalol produced a rise in fibrillation threshold that occurred simultaneously with a prolongation in monophasic action potential duration and effective refractory period of the contractile fibres and a slowing in fibrillation rate, whereas conduction time was not affected. The changes appeared, however, to be less pronounced in the ventricles than in the atria, in which vulnerability to fibrillation, normally increased by vagal tone, had been previously enhanced by acetylcholine. Sotalol antagonised the changes due to acetylcholine. In both the atria and the ventricles the first dose (1 mg X kg-1), which produced plasma concentrations of approximately 2 micrograms X ml-1 10 min after injection, produced a submaximal effect. Nevertheless, subsequent administrations increased the beneficial effects but not in proportion to the dose and plasma concentrations.