The effect of α1-adrenoceptor stimulation on the delayed rectifier K+current (Ik) was examined in isolated guinea pig ventricular cells by use of the patch-clamp method. IKwas evoked by a 3-second depolarizing pulse from a holding potential of −30 mV in a Na+- and K+-free solution containing 3 μM nifedipine. Phenylephrine (30 μM) in the presence of propranolol (1 μM) produced an increase in IK. In five cells, phenylephrine increased the tail current of IKby 23±5%. This effect of phenylephrine was blocked by prazosin (0.3 μM), a selective α1-blocker. Phenylephrine produced only a small effect on the voltage and time dependence of IK. Pretreatment with 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7, 10 μM) abolished the phenylephrine-induced increase in IK. In addition, pretreatment with a maximally effective concentration of 12-O-tetradecanoylphorbol 13-acetate (100 nM) abolished the phenylephrine-induced increase in IK. In conclusion, α1-adrenoceptor stimulation increases IKin guinea pig cardiomyocytes. This cvl-adrenoceptor-mediated response may be related to an activation of protein kinase C. The increase in IKmay explain a shortening of action potential duration observed after α1-adrenoceptor stimulation in guinea pig cells.