In this study, we provide evidence for the operation of BNIP3 as a key regulator of mitochondrial function and cell death of ventricular myocytes during hypoxia. In contrast to normoxic cells, a 5.6-fold increase (P<0.05) in myocyte death was observed in cells subjected to hypoxia. Moreover, a significant increase in BNIP3 expression was detected in postnatal ventricular myocytes and adult rat hearts subjected to hypoxia. An increase in BNIP3 expression was detected in adult rat hearts in vivo with chronic heart failure. Subcellular fractionation experiments indicated that endogenous BNIP3 was integrated into the mitochondrial membranes during hypoxia. Adenovirus-mediated delivery of full-length BNIP3 to myocytes was toxic and provoked an 8.3-fold increase (P<0.05) in myocyte death with features typical of apoptosis. Mitochondrial defects consistent with opening of the permeability transition pore (PT pore) were observed in cells expressing BNIP3 but not in cells expressing BNIP3 missing the carboxyl-terminal transmembrane domain (BNIP3ΔTM), necessary for mitochondrial insertion. The pan-caspase inhibitor z-VAD-fmk (25 to 100 μmol/L) suppressed BNIP3-induced cell death of ventricular myocytes in a dose-dependent manner. Bongkrekic acid (50 μmol/L), an inhibitor of the PT pore, prevented BNIP3-induced mitochondrial defects and cell death. Expression of BNIP3ΔTM suppressed the hypoxia-induced integration of the endogenous BNIP3 protein and cell death of ventricular myocytes. To our knowledge, the data provide the first evidence for the involvement of BNIP3 as an inducible factor that provokes mitochondrial defects and cell death of ventricular myocytes during hypoxia.