1. In the present work, the effects of the antiarrhythmic drug, berberine, on action potential and ionic currents of cat ventricular myocytes were studied. 2. Berberine prolonged action potential duration in cat ventricular myocytes without altering other variables of the action potential. 3. The drug at concentrations of 0.3-30 microM blocked only the delayed rectifier (IK) current with an IC50 = 4.1 microM. Berberine produced a tonic block and a phasic block that was increased with the duration of the depolarizing pulse. The blocking effect on IK was use-dependent, but not frequency-dependent. 4. In cardiac preparations two delayed rectifier currents have been found: a rapid (IKr) current and a slow (IKs) current. In the present work it has been found that berberine at the concentrations used, selectively blocked IKr. 5. At concentrations higher than 10 microM it also decreased the transient outward (Ito1) current. The drug did not have effects on the inward rectifier (IK1) or the high threshold calcium current (Ica-L). 6. These results show that berberine is a specific potassium channel blocker. The increase in action potential duration induced by berberine can be explained mainly by its blocking effects on IK.