AIM: To study apoptotic injury induced by aeactive oxygen species - hydrogen peroxide (H2O2) on cardiac myocytes. METHODS: Cultured rat neonatal cardiac myocytes were treated with H2O2 of various concentra- tion to observe apoptoitic injury of cardiomyocytes by agarose gel electrophoresis, Giemsa - stained smears of cell, and flow cytometry, meanwhile lactate dehydrogenas (LDH) and malondialdehyde (MDA) were determined to assess the effect of H2O2 on lipid peroxidation and permeability of the plasma membrane. RESULTS: 5 mmol/L H2O2 caused culted cardiomyocytes apoptotic morophological characteristics, including nucleosomal DNA fragmentation in my- ocytes by agarose gell electrophoresis (DNA ladder), cell shrinkage, nuclear condensation, and chromatin margin by Giemsa-stained cell smears, and aneuploid peak (AP) - apoptotic bodies occurence by flow cytometry. CONCLU- SONS: H2O2 - induced apotosis in myocytes was a the - and concentration - dependent process, Treatment with low concentration of H2O2(10 mmol/L) rapidly induced a necrotic form of death characterized by smeared patterns of DNA digestion on agarose gel electrophoresis and lethal membrane disuption (as measured by LDH release). Exposure of 5 - 10 mmol/ L H2O2 induced cardiomyocytes apoptosis concurrently with biochemical changes of LDH and MDA increase in the medium.