An autoradiographic technique adapted to diffusible compounds was used to localize steroid receptor sites in various tissues of the chicken embryo. In this article are presented results obtained on the gonads, the reproductive tract, the bursa of Fabricius, and the anterior pituitary after injection of either 3H-estradiol or 3H-dihydrotes-tosterone into 5 1/2- to 15-day-old chicken embryos. Target cells for steroid hormones, either estrogen or androgen, or both, are present in these organs early in the development. The precocity of the receptors suggests that sex steroids may influence embryonic differentiation earlier than currently recognized. The presence of the receptors in a variety of organs or tissues, known or unsuspected target for steroids, emphasizes the very diversified roles of sex steroid hormones in the differentiation processes. Conditions for these receptors to be active are discussed with respect to particularities of the embryonic system.