The electrophysiology of neonatal rat ventricular myocytes with and without hypertrophy has not been characterized. The ₁-adrenergic agonist phenylephrine induced hypertrophy in neonatal rat ventricular myocytes. After 48 h of exposure to 20 µM phenylephrine, cell surface area of hypertrophied myocytes was 44% larger than control. Action potential duration was significantly longer in hypertrophy than in control. There was an increase in L-type Ca²⁺ current in control after 48 h in culture, but current density was significantly less in hypertrophy (4.7 ± 0.8 hypertrophy vs. 10.7 ± 1.2 control pA/pF, n = 22, P < 0.05). T-type Ca²⁺ current density was not different. The -adrenergic antagonist prazosin blocked the hypertrophy and the chronic effect of phenylephrine on L-type Ca²⁺ current. Transient outward K⁺ current density was decreased 70% in hypertrophy and was blocked with 4-aminopyridine. No change in Na⁺ current density was observed. Staurosporine, a protein kinase C inhibitor, eliminated the hypertrophy and the effect on L-type Ca²⁺ current. These studies showed that phenylephrine-induced hypertrophy occurred via the ₁-adrenergic pathway and caused electrophysiological changes and effects on ion channel expression.