Norepinephrine (NE) causes hypertrophic growth of cardiac myocytes via stimulation of alpha₁-adrenergic receptors (α₁-AR). Reactive oxygen species (ROS) can act as signaling molecules for cell growth. Accordingly, we tested the hypothesis that ROS mediate α₁-AR-stimulated hypertrophic growth in adult rat ventricular myocytes (ARVM). NE increased the level of intracellular ROS as assessed by lucigenin chemiluminescence or cytochrome c reduction, and this effect was prevented by the superoxide dismutase (SOD)-mimetic MnTMPyP. NE also caused the induction of MnSOD mRNA. α₁-AR stimulation with NE (1 μM ) in the presence of propranolol (2 μM ) for 48–96 h caused a hypertrophic growth phenotype characterized by a 36±3% increase in³H-leucine incorporation, a 49±14% increase in protein accumulation, a six-fold induction of atrial natriuretic peptide mRNA, actin filament reorganization, and the induction of MnSOD mRNA. These responses were all prevented by pretreatment with the α₁-AR-selective antagonist prazosin (100 nM ) or the SOD-mimetics MnTMPyP (50 μM ) and Euk-8 (100 μM ). MnTMPyP had no effect on α₁-AR-stimulated³H-inositol phosphate turnover or the hypertrophic phenotype caused by the protein kinase C activator phorbol-12-myristate-13-acetate. Thus, ROS play a critical role in mediating the hypertrophic growth response to α₁-AR-stimulation in ARVM.