The effects of amoxapine on membrane potentials and membrane currents of rabbit sinoatrial node were studied using the double microelectrode voltage clamp method. Amoxapine (greater than 1 mumol.litre-1) decreased the heart rate and the maximum rate of rise and the rate of diastolic depolarisation in a dose dependent manner. Above 3 mumol.litre-1, amoxapine also decreased the action potential amplitude and prolonged the action potential duration at half amplitude. These electrophysiological changes induced by amoxapine were relatively reduced in a high calcium medium (extracellular calcium concentration 4.0 mmol.litre-1). In voltage clamp experiments amoxapine depressed the slow inward current, the time dependent potassium current, and the hyperpolarisation activated inward current. The major effect, however, was considered to be a reduction of the slow inward current. It is concluded that amoxapine produced an inhibitory action on the electrical activity of sinoatrial node, and this action is mainly explained by an inhibition of calcium influx through the cell membrane.