Summary. From hatching to sexual maturity, the left ovary of the chick was studied with cytochemical techniques for the demonstration of steroid synthesis. At the time of hatching, there are in the ovarian medulla cell groups loaded with lipids and cholesterol which give a positive reaction for Ah-3fl-hydroxysteroid dehydrogenase (Ah-3fl-HSDH). Because of the predominant growth of the cortex in the course of the postnatal morphogenesis of the ovary, these cells are shifted to the theca interna of the follicles and the interfollicular spaces (interstitial cells). The cytochemical tests used demonstrate that, at all the stages studied, there is steroid synthesis in these cells. From the 20th day on, granulosa cells show an unspecific enzymatic reaction, which is positive in the control sections incubated without substrate. A strong As-3fl-HSDH activity is observed in the granulosa cells of postovulatory follicles. The significance of these findings in relation to the life history of steroid producing cells in the embryonic and postnatal chick ovary is discussed.