The neurotoxin 6-hydroxydopamine (6-OHDA) induces apoptosis in the rat phaeochromocytoma cell line PC12. 6-OHDA-induced apoptosis is morphologically indistinguishable from serum deprivation-induced apoptosis. Exposure of PC12 cells to a low concentration of 6-OHDA (25 microM) results in apoptosis, whereas an increased concentration (50 microM) results in a mixture of apoptosis and necrosis. We investigated the involvement of caspases in the apoptotic death of PC12 cells induced by 6-OHDA, using a general caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (zVAD-fmk), and compared this with serum deprivation-induced apoptosis, which is known to involve caspases. We show that zVAD-fmk (100 microM) completely prevented the apoptotic morphology of chromatin condensation induced by exposure to either 6-OHDA (25 and 50 microM) or serum deprivation. Furthermore, cell lysates from 6-OHDA-treated cultures showed cleavage of a fluorogenic substrate for caspase-3-like proteases (caspase-2, 3, and 7), acetyl-Asp-Glu-Val-Asp-aminomethylcoumarin, and this was inhibited by zVAD-fmk. However, although zVAD-fmk restored total cell viability to serum-deprived cells or cells exposed to 25 microM 6-OHDA, the inhibitor did not restore viability to cells exposed to 50 microM 6-OHDA. These data show the involvement of a caspase-3-like protease in 6-OHDA-induced apoptosis and that caspase inhibition is sufficient to rescue PC12 cells from the apoptotic but not the necrotic component of 6-OHDA neurotoxicity.