BACKGROUND: The possible deleterious effects of low-grade, chronic environmental and occupational exposure to organophosphorus compounds (OPCs) are not well documented. OBJECTIVE: To investigate the possible effects of low-level, chronic exposure of pregnant mothers to OPCs on the fetus by measuring OPC levels, and using markers of OPC exposure, oxidative stress and oxidative tissue damage. METHODS: Toxicity was assessed by measuring (i) OPC levels in breast milk and plasma from maternal and cord blood using gas chromatography, (ii) maternal and fetal butyrylcholinesterase (BChE) activity using inhibition assays, (iii) antioxidant status of the fetus using superoxide dismutase activity assays, (iv) oxidative stress in the fetus by determining malondialdehyde (MDA) concentrations, and (v) examining for fetal DNA fragmentation using electrophoresis. Samples were obtained from consenting mothers living in a farming community in southern Sri Lanka at the end of the pesticide spray season (study group) and just before the commencement of the spray season (in-between spray season; control group). RESULTS: Organophosphate residues were detected in only two subjects (chlorpyrifos in maternal and cord blood of one during the spray season and dimethoate in breast milk of another during the in between spray season), but the test employed was capable of only detecting concentrations above 0.05 mg/l. However, cord blood obtained during the spray season showed significant inhibition of BChE activity, increased oxidative stress and more DNA fragmentation when compared with cord blood obtained during the in-between spray season. CONCLUSIONS: Inhibition of cord blood BChE activity indicates fetal exposure to organophosphorus compounds during times when there is a high probability of environmental drift. This provides a plausible explanation for the increased oxidative stress and high DNA fragmentation in the fetus. Long-term outcomes of such exposures are unknown.