Plasma estrone (E1) concentrations in male and female chick embryos were determined by radioimmunoassay (RIA) on days 9.5, 11.5, 12.5, 13.5, 14.5, 15.5, and 17.5 of development. Levels of E1 in female embryos demonstrate a statistically significant increase from days 9.5 (275.0 pg/ml) to 13.5 (399.0 pg/ml). On days 14.5 to 17.5 plasma E1 values appear to decrease with respect to day 13.5 values (day 14.5, 349.0 pg/ml; day 15.5, 326.0 pg/ml; day 17.5, 338.6 pg/ml). This change, however, may or may not be real (P less than .06). In male embryos, plasma E1 concentrations remain at a relatively steady level throughout development (day 9.5, 184.4 pg/ml to day 17.5, 230 pg/ml) and are statistically significantly lower than those of female embryos. Results are discussed in terms of the possible role of E1 in the growth and differentiation of accessory and secondary sex characters.