Chick embryos were injected on incubation Day 14 with 62 microliter of triorthocresyl phosphate (TOCP)/kg egg. Muscles of the leg were examined from 5 to 25 days after hatching. The sartorius from the thigh and the external gastrocnemius and peroneus longus from the tibial leg region were compared for muscle fiber size and end-plate length over this period. Treated chicks showed no acute toxic effects or overt ataxia and were equal in body weight to controls. At 5, 15, and 25 days after hatching, morphologic alterations consistent with denervation were detected. Muscle fibers were smaller than controls on Day 5 and were hypertrophic on Days 15 and 25. On Day 5 growth of fibers was retarded, an effect consistent with denervation, and the subsequent hypertrophy is predicted as compensation for denervated fibers. Small end plates were seen on Day 15, characteristic of end plates that were delayed in development by denervation. Each of these differences was greater in the tibial muscles than in the more proximally located sartorius. This is consistent with a distal neuropathy, such as that caused by TOCP in adult hens. Some recovery was apparent at the low dose 25 days after hatching. It is suggested that this resulted from reinnervation by repaired axons. This study of the myoneural apparatus and muscle fiber response to TOCP adds evidence to the possibility that the developing chick embryo may develop delayed neuropathy from organophosphorus compounds which produce this effect in adult hens.