AbstractThe effect of Azodrin on avian development was studied using a bobwhite quail line and two chicken lines—a single comb White Leghorn (SCWL) and an Australorp line. The bobwhite quail embryos did not respond to injections of Azodrin until stages 22 to 23 (6 days of incubation); the SCWL embryos, not until stage 18 (3 days of incubation) with increasing susceptibility through stage 20. The threshold concentration, at stage 19, for the SCWL was 0.4 mg/kg, for the Australorp, less. Within one hour of treatment stage‐20 embryo shape was altered. There was, however, no immediate response from embryos treated earlier. The effect appeared as a reduced growth rate in the cervical flexure, and may be mediated by interference with the normal energy balance. The response, once initiated, was continuous through 10 days of incubat