Apoptosis via tau phosphorylation has been implicated in the selective neuronal losses seen in Alzheimer's disease (AD). Previous studies in vivo and in cultured neurons have shown that okadaic acid (OA) evokes tau phosphorylation to initiate a neurodegeneration that resembles the pathogenesis of AD. In an effort to identify additional key molecules in this neurodegeneration, we treated cultured rat neurons with OA and examined the apoptosis-related effects, such as changes in mitochondrial activity and expression levels of JNK, Bim, Bad, Bax and caspase-3. Western blotting revealed that phosphorylation of JNK and c-jun occurred first, followed by increased expression of Bim and subsequent caspase-3 activation in OA-treated neurons. In contrast, Bad levels decreased as early as 4 h after OA treatment. Immunocytochemistry showed that the increased phospho-JNK immunoreactivity was localized in the cytosol of degenerating neurons, while increased phospho-c-jun was localized in the nucleus. The mitochondria showed decreased membrane potential and increased swelling after OA treatment. Collectively, these data suggest that JNK- and Bim-related mitochondrial dysfunction is involved in OA-induced neurodegeneration.