Overexpression of the c-myc protooncogene in the heart of transgenic mice has been demonstrated to result in cardiac enlargement due to increased myocyte hyperplasia during the fetal period. To determine the age of completion of the proliferative phase of myocyte growth in neonatal mice with c-myc overexpression, we used a transgenic (TG) mouse model in which c-myc overexpression is limited to the heart. Bromodeoxyuridine (BrdU) was given to TG and wild type (WT) mice (n=3/group) at 1, 2, 3, 5, 7, 10, 14, 16, 18 and 20 days of age to identify cells in S-phase of the cell cycle. Increased cardiac mass was present in TG compared to WT mice at all time periods (P<0. 05). Using computer assisted image analysis, myocardial total nuclear density (NT) in TG mice was 7-31% greater in both the left ventricle (LV) and the interventricular septum (IVS) than in WT at all ages (P<0.05), indicative of a smaller myocyte size. In WT mice, the labeling index (LI) remained almost constant at approximately 11-12% until 7 days of age, and then rapidly dropped to approximately 2% by 14 days and to less than 1% by 20 days. In contrast, LI in TG dropped continuously from birth to approximately 4% at 7 days and approximately 2% at 10 days of age (P<0.001). Thus, overexpression of the c-myc protooncogene is associated with enhanced hyperplastic growth of the heart during fetal development, and accelerated neonatal conversion to hypertrophic myocyte growth.