Insulin-like growth factor I (IGF-I) is an important survival growth factor that has been shown to inhibit apoptosis, but the effects of IGF-I on apoptotic signaling remain largely unknown. To investigate IGF-I actions on apoptosis of H9C2 cardiac muscle cells, we have defined the effects of IGF-I on Bcl-2, Bax, caspase 3, DNA fragmentation, and cell survival. The abundance of Bcl-2 and Bax was determined with immunoblotting, and the activities of caspase 3 were assayed with the labeled substrate DEVD-p-nitroanilide. The occurrence of apoptosis was determined by electrophoresis of labeled DNA fragments and by in situ terminal deoxynucleotidyl transferase UTP nick end labeling assay. We found that apoptosis of H9C2 cells, induced by serum withdrawal and doxorubicin, was associated with the induction of Bax and the activation of caspase 3. IGF-I partially inhibited Bax induction, caspase 3 activation, DNA fragmentation, and enhanced cell survival. Interestingly, there is a compensatory rise in the abundance of Bcl-2 upon serum withdrawal and doxorubicin treatment, and IGF-I stimulation resulted in decreased induction of Bcl-2. These results suggest that serum withdrawal- and doxorubicin-induced apoptosis of H9C2 cells probably in part resulted from induction of Bax and caspase 3, and IGF-I inhibited apoptosis by attenuating Bax induction and caspase 3 activation.