Eight chemicals that are cytidine analogues or nucleosides (5-azacytidine (5AzC), 5-azadeoxycytidine, 6-azacytidine, 5-azacytosin, cytidine, 3-deazaadenine, 3-deazauridine and 6-azauridine) were examined for the ability to induce neuronal apoptosis. 5AzC and 5-azadeoxycytidine induced apoptosis in the brain and spinal cord of the fetuses at 24 hr after the injection to dams, while the other chemicals tested failed to induce apoptosis. In the system of PC12 cells, only 5AzC induced apoptosis, and other chemicals failed to provoke morphological and biochemical changes characteristic of apoptosis. 5AzC, 5-azadeoxycytidine and 6-azacytidine failed to induce apoptosis in C6 cells. Gel electrophoresis after MspI or HapII digestions revealed no apparent evidence of DNA demethylation after 5AzC-treatment in either fetal brains or PC12 cells. These results indicate that DNA demethylation is possibly involved in 5AzC-induced neuronal apoptosis although no direct evidence of DNA demethylation was obtained.