Effects of phorbol esters on spontaneously beating rabbit sino-atrial (SA) node cells were investigated by means of voltage clamp technique. In a small SA node specimen, 12-O-tetradecanoylphorbol-13-acetate (TPA) 10(-7) mol/l lengthened the cycle length (CL) and at over 3 X 10(-7) mol/l prolonged the action potential duration (APD). Action potential amplitude (APA), maximum diastolic potential (MDP) and maximum rate of rise (Vmax) were unaffected. Amiloride 10(-3) mol/l, an inhibitor of Na+-H+ exchange, did not reverse the phorbol ester-induced effects. In voltage-clamp experiments, TPA 1-10 X 10(-7) mol/l slightly increased the slow inward current (Isi) and the time-dependent inward current (Ih) which activates during hyperpolarization. The outward current and the tail current were reduced, although the activation curve was not shifted along the voltage axis. In the presence of 10(-7) mol/l isoprenaline, TPA produced dysrhythmia and a transient inward current in voltage-clamp experiments. In the presence of 5 X 10(-5) mol/l phenylephrine or 2 X 10(-6) mol/l acetylcholine, TPA also elicited dysrhythmia. 4-beta-phorbol-12,13-dibutyrate (PBD) induced similar electrophysiological effects as TPA, but 4-alpha-phorbol-12,13-didecanoate (PDD) never did so even in the presence of isoprenaline. These results suggest that TPA and PDB might mobilize intracellular Ca2+ via protein kinase C activation in the presence of isoprenaline, phenylephrine or acetylcholine, resulting in dysrhythmia due to delayed afterdepolarization.