Teratogenic effects of two organophosphate insecticides, diazinon and dicrotophos, were investigated in regard to skeletal development, particularly of the extremities and vertebrae. Cartilage and calcified bone were examined with alcian blue and alizarin red S staining techniques, respectively, in chick embryos of d 5 to 17 of incubation. The age-related development of both cartilaginous and ossified portions of the hind leg was measured in control and insecticide-treated groups. Diazinon and dicrotophos (200 micrograms/egg), injected on d 3, inhibited growth of the following skeletal elements: femur, tibia, metatarsi and digits of the leg. The inhibition was noticeable from the 9th d of incubation. The greatest reduction of the skeletal length was observed in tibia and metatarsi, and was characterized by angulations toward the dorsal side. Percent of growth inhibition of the calcified region in the legs was similar to that of the entire length of each skeletal element, but there was no difference between control and insecticide-treated groups on the time-related appearance of cartilaginous or calcified long bones, digits, and phalanges of legs. In the cervical region of embryos treated with the insecticides, unique deformities such as an "undulating" notochord and fused cervical rings were seen at an early stage (d 6). We suggest that the organophosphate-induced malformations in legs are mainly due to growth retardation of later stages of development of each skeletal element. On the other hand, the neck deformities result from a profound alteration of differentiation at early stages of development.