We previously reported that the morphine alkaloid derivative buprenorphine hydrochloride (Bph) induces rapid apoptosis in NG108-15 nerve cells accompanied by the activation of caspase-3. Here, we found this kind of apoptosis was also accompanied by rapid loss of the mitochondrial membrane potential, followed by the efflux of cytochrome c from the mitochondria to the cytosol and the activation of caspases-9 and -3. Together, these results strongly suggested the Bph death signal was routed through the mitochondrial pathway in NG108-15 cells. In these cells, serum-starvation induces a different apoptosis, which we exploited to investigate Bcl-2's role as an apoptosis inhibitor. We made an NG108-15 transfectant, Bcl-2(P2), that stably expressed human Bcl-2, and used it to test Bcl-2's effect on the serum-starvation-induced apoptosis in NG108-15 cells. Cell viability, DNA-ladder formation, and efflux of cytochrome c from the mitochondria were all detected, showing that the human Bcl-2 functioned normally in the Bcl-2(P2) cells. Although the apoptotic events tested were identical in the parental cells and transformants, Bcl-2 expression completely failed to inhibit Bph-induced apoptosis in the Bcl-2(P2) cells.