The effects of pregnancy and lactation on the toxicity and distribution of parathion and paraoxon were examined. Signs of cholinergic stimulation were more intense in pregnant mice when compared to virgin controls after administration of parathion or its active metabolite, paraoxon. Cholinesterase activity and tissue levels of parathion and paraoxon were determined in mice at 19 days of gestation or Day 19 postpartum after administration of a single dose of 5 mg/kg parathion or 0.58 mg/kg paraoxon. Plasma (pseudo) cholinesterase activity was consistently lower in treated pregnant mice. Total brain cholinesterase was also suppressed to a greater degree in pregnant mice after treatment with parathion or paraoxon when compared with virgin animals treated similarly. In addition, when equal quantities of paraoxon (32 micrograms) were administered to both pregnant and virgin animals, total brain cholinesterase was significantly less in pregnant mice. Administration of parathion to lactating mice on Day 19 postpartum did not result in any significant differences in plasma or brain cholinesterase activity when compared to that in virgin animals. Pregnant mice treated with 5 mg/kg parathion demonstrated higher concentrations of both parathion and paraoxon in blood and brain than similarly treated virgin controls which correlated with the enhanced cholinesterase inhibition. Decreased ability to detoxify paraoxon was also demonstrated by a significant reduction in serum paraoxonase activity during pregnancy.