Japanese quail eggs were injected with estradiol benzoate (EB) after 10, 11, 12, 13, or 14 days of incubation, or were injected on day 10 with one of six dosages of EB or seven dosages of testosterone propionate (TP). EB injected on day 10 of 11 demasculinized the males; as adults they were incapable of copulating in response to exogenous TP. EB injected on day 13 or 14 was without effect, and EB injected on day 12 caused slight demasculinization. The critical period for sexual differentiation of the brain appears to end after about 12 days of incubation in this species. As little as 1 microgram EB reduced masculine sexual behavior. Injection of TP at dosages equal to or greater than 100 micrograms also demasculinized male quail.