Though little attention has been given to the possibility that glial cells may represent a target for the developmental neurotoxicity of organophosphorus (OP) insecticides, recent evidence, obtained in particular with chlorpyrifos (CP), suggests that developmental exposure to this compound may indeed target astrocyles. To substantiate and expand these observations, we carried out a series of in vitro studies utilizing fetal rat astrocytes and a human astrocytoma cell line. 1321N1 cells, to investigate the effect of the OPs CP, diazinon (DZ) and parathion (P), their oxygen analogs chlorpyrifos oxon (CPO), diazoxon (DZO) and paraoxon (PO), and their metabolites 3,5,6-trichloro-2-pyridinol (TCP), 2-isopropyl-6-methyl-4-pyrimidoI (IMP) and para-nitrophenol (PNP), on cell proliferation. In fetal rat astrocytes and astrocytoma cells maintained in serum, CP, DZ, P, CPO, DZO, and PO induced a concentration-dependent inhibition in [3 H]thymidine incorporation with a very similar potency (IC50 between 45 and 57 mu M). Among the other metabolites, PNP was the most potent (IC50 = 70-80 mu M), while TCP and IMP were much less effective (IC50 > 100 mu M)Cytotoxicity appears to account only for a small part of the effect on DNA synthesis. OP insecticides and their oxons were three-to six-fold more potent in inhibiting [H-3]thyrpidine incorporation when cells were synchronized in the G(0)/G(1) phase of the cell cycle and re-stimulated by carbachol or epidermal growth factor. These results suggest that OP insecticides and their G(0)/G(1) to the S/G(2) phase of the cell cycle may be particularly sensitive to the action of these compounds.