beta-Adrenergic stimulation of the heart is thought to increase cardiac muscle contractility by activation of cyclic AMP-dependent protein kinase and concomitant increase in the phosphorylation of certain proteins (for refs see refs 1-6). Electrophysiological studies have shown that the stimulation of cardiac beta-adrenoreceptors, the external application of cyclic AMP or its analogues to Purkinje fibres, or the injection of cyclic AMP into single myocytes can increase the slow inward current (Isi) during the plateau phase of the action potential (AP). In heart muscle this current is mainly carried by Ca2+ (refs 10, 11) and it has been suggested that cyclic AMP-dependent phosphorylation of some component of the calcium channel increases the amount of Ca2+ which enters the cell during depolarization. We have investigated this hypothesis by examining the electrical responses of isolated guinea pig ventricular myocytes to pressure injections of subunits of the cyclic AMP-dependent protein kinase. We report here that injection of the catalytic subunit (C) resulted in a lengthening of the action potential duration (APD) and an increase in the height of the plateau as well as the amplitude of Isi. By contrast, the injection of regulatory subunit (R) shortened the APD of fast and slow response APs, an effect which was reversed by adrenaline.