Avian species follow the ZW/ZZ system of sex determination, which the female is heterogametic and expresses H-Y (or, more appropriately, 'H-W') antigen. We present the results of an investigation into the effects of the antiestrogen, tamoxifen, on gonadal differentiation and H-Y antigen expression in chickens. When given at doses of 0.25-2 mg per egg immediately before incubation, tamoxifen blocked regression of the right gonad in a significant number of 14-day-old female embryos. The nonregressed right gonad had a testis-like external appearance and, in some cases, contained what appeared to be spermatogenic tubules. Tamoxifen had no histologically detectable effect on the differentiation of the left ovary or the testes. In spite of tamoxifen's histological effects on right female gonads, it did not masculinize the steroidogenic capabilities of these gonads. Whether obtained from drug- or vehicle-treated embryos, the left and right female gonads always contained appreciable amounts of estrogen. In contrast, testes obtained from either drug- or vehicle-treated embryos did not contain detectable amounts of estrogen. Tamoxifen reduced the H-Y antigen levels in female liver and gonads. In both left and right female gonads, the reduction was to male levels. In female livers, tamoxifen reduced H-Y antigen to levels intermediate between those of normal males and females. Thus, the expression of H-Y antigen in both gonadal and nongonadal tissue is estrogen dependent, but the dependency appears to be more stringent for gonadal tissue.(ABSTRACT TRUNCATED AT 250 WORDS)