Diazinon and cypermethrin are pesticides extensively used in sheep dipping. Diazinon is a known anti-cholinesterase, but there is limited information regarding its molecular mechanism of action. This paper describes the effects of diazinon and cypermethrin at a morphological and molecular level on differentiating mouse N2a neuroblastoma and rat C6 glioma cell lines. Concentrations up to 10 microM of both compounds and their mixture had no effect on the viability of either cell line, as determined by methyl blue tetrazolium reduction and total protein assays. Microscopic analysis revealed that 1 microM and 10 microM diazinon but not cypermethrin inhibited the outgrowth of axon-like processes in N2a cells after a 24-h exposure but neither compound affected process outgrowth by differentiating C6 cells at these concentrations. Under these conditions, 10 microM diazinon inhibited AChE slightly compared to the control after a 4-h exposure but not after 24 h. Western blotting analysis showed that morphological changes were associated with reduced cross-reactivity with antibodies that recognize the neurofilament heavy chain (NFH), microtubule associated protein MAP 1B and HSP-70 compared to control cell extracts, whereas reactivity with anti-alpha-tubulin antibodies was unchanged. Aggregation of NFH was observed in cell bodies of diazinon-treated N2a cells, as determined by indirect immunofluorescence staining. These data demonstrate that diazinon specifically targets neurite outgrowth in neuronal cells and that this effect is associated with disruption of axonal cytoskeleton proteins, whereas cypermethrin has no effect on the same parameters.