Injection of chlorophos (300 mg/kg, i. m.) to rats produced after 4 hours an elevation of phospholipid level in liver mitochondria by 35.8% on an average. The central cholinolytic cyclosil did not prevent the effect from being realized. Chlorophos (10(-7)-10(-5)M) in vitro failed to influence the phospholipid level in isolated mitochondria but their content in intact mitochondria was appreciably diminished after incubation. The rate of phospholipid biosynthesis in the mitochondrial membranes was not affected by chlorophos poisoning. It is suggested that the effect of chlorophos is mediated by its interaction with mitochondrial phospholipases.