The aim of this work was to study the neurodegenerative effects of the organophosphate (OP) pesticides chlorpyrifos (CPF) and chlorpyrifos-methyl (CHM) on cultured mouse N2a neuroblastoma cells. CPF or CHM, at a subcytotoxic concentration of 3 muM, were added to the cells either at the time of the induction of cell differentiation (codifferentiation) or 16 h after the induction of differentiation (postdifferentiation). CPF and CHM were similar in inhibiting significantly the outgrowth of axon-like processes from N2a cells after only 4 h exposure under both co- and postdifferentiation exposure conditions. Densitometric scanning of Western blots of extracts of cells treated with CPF or CHM for 4 h revealed significantly decreased cross-reactivity with a monoclonal antibody recognizing the protein GAP-43 under post- but not under codifferentiation exposure conditions. Exposure to CPF or CHM for 4 h under postdifferentiation conditions also resulted in reduced fluorescence of N2a cell body staining with anti-GAP-43. Cross-reactivity of Western blots with a monoclonal antibody recognizing alpha-tubulin was not significantly affected by OP treatment. These data indicate that a disturbance in GAP-43 may be important in the retraction of axons in predifferentiated N2a cells and support the notion that the mechanisms involved in CPF-and CHM-induced inhibition of axonal outgrowth may be different under co- and postdifferentiation exposure conditions.