Chronic complex I inhibition caused by rotenone induces features of Parkinson's disease in rats, including selective nigrostriatal dopaminergic degeneration and Lewy bodies with alpha-synuclein-positive inclusions. To determine the mechanisms underlying rotenone-induced neuronal death, we used an in vitro model of human dopaminergic SH-SY5Y cells. In rotenone-induced cell death, rotenone induced Bad dephosphorylation without changing the amount of Bad proteins. Rotenone also increased the amount of alpha-synuclein in cells showing morphological changes in response to rotenone. Because Bad and alpha-synuclein are known to bind to 14-3-3 proteins, we examined the effects of rotenone on these complexes. Whereas a decreased Bad amount bound to 14-3-3 proteins, rotenone increased alpha-synuclein binding to these proteins. Because dephosphorylation by calcineurin activates Bad, we examined the possible involvement of Bad activation in rotenone-induced apoptosis by using the calcineurin inhibitor tacrolimus (FK506). Tacrolimus suppressed two rotenone-induced actions: Bad dephosphorylation and apoptosis. Furthermore, the inhibition of caspase-9, which functions downstream from Bad, completely suppressed rotenone-induced apoptosis. Our findings demonstrate that Bad activation plays a role in rotenone-induced apoptosis of SH-SY5Y cells.