Reactive oxygen species (ROS) can act as signaling molecules to stimulate either hypertrophy or apoptosis in cardiac myocytes. We tested the hypothesis that the phenotypic effects of ROS are due to differential, concentration-dependent activation of specific kinase signaling pathways. Adult rat ventricular myocytes were exposed to H₂O₂ over a broad concentration range (10–1000 μM). Low concentrations of H₂O₂ (10–30 μM) increased protein synthesis without affecting survival. Higher concentrations of H₂O₂ (100–200 μM) increased apoptosis (assessed by TUNEL). Still higher concentrations of H₂O₂ (300–1000 μM) caused both apoptosis and necrosis. A hypertrophic concentration of H₂O₂ (10 μM) increased the activity of ERK1/2, but not that of JNK, p38 kinase or Akt. An apoptotic concentration of H₂O₂ (100 μM) activated JNK, p38 kinase and Akt, and further activated ERK1/2. The MEK1/2 inhibitor U0126 prevented the hypertrophic effect of 10 μM H₂O₂. The apoptotic effect of 100 μM H₂O₂ was inhibited bya dominant-negative JNK adenovirus, and was potentiated by U0126 or an Akt inhibitor. Thus, the concentration-dependent effects of ROS on myocyte hypertrophy and growth are due, at least in part, to the differential activation of specific kinase signaling pathways that regulate hypertrophy and apoptosis.