The purpose of this study was to evaluate the toxicity of diazinon oxon (DZO), a major in vivo metabolite of the organophosphate insecticide diazinon (DZ), on differentiating rat C6 glioma cells. At concentrations shown to be non-cytotoxic by both the MTT and the Kenacid blue dye binding assays (1, 5 and 10 microM), DZO caused after 24h a reduction in the number of extensions developed from C6 cells induced to differentiate by serum withdrawal and addition of sodium butyrate. Densitometric scanning of Western blots of extracts of C6 cells demonstrated that, at all concentrations used, DZO decreased after 24h the expression of glial fibrillary acidic protein (GFAP) compared to controls. In addition, exposure to 10 microM DZO for 24h reduced the levels of tubulin and microtubule associated protein 1B (MAP1B). On the other hand, levels of MAP2c were not affected by DZO treatment. In contrast to our previous data on DZ, the above findings suggest that its oxon metabolite, DZO, may, at biologically relevant, subcytotoxic concentrations, interfere with glial cell differentiation.