It has been established that differentiation of the female gonad of the chicken is different in the right and left ovaries. Whereas the left ovary differentiates into the adult functional ovary, the right one suffers regressive changes and does not participate in the production of ova. Hence, we considered of importance to study the structural, ultrastructural and cytochemical characteristics of interstitial cells, which secrete estrogens in the left ovary and in the right one that undergoes atrophy, which has not been yet analyzed in this aspect. Chick embryos at 7, 11, 15 and 19 days of incubation were employed. The techniques used were H/E, Sudan IV, 3 -beta- hydroxysteroid dehydrogenase and transmission electron microscopy. At the light microscope these cells were clearly identified in the medullary cords, grouped in nests circumscribed by connective tissue, since 11 days of incubation. With H/E they exhibited a cytoplasm of foamy aspect and clear vacuoles of variable size. The amount of cell nests increased with the ages of the embryos. Besides, we found accumulation of sudanophilic granules and an enzymatic activity that increased during the course of development. At the ultrastructural level these cells contained an abundant smooth endoplasmic reticulum with tubules and vesicles, mitochondrias with tubular cristae and a considerable number of lipidic inclusions. The above mentioned elements confer to these cells and steroid-secreting activity. It is noteworthy that in the ovary that regresses interstitial cells do not disappear. In conclusion, we may point out that the right ovary would be rudimentary in ova formation, but would remain active with respect to hormonal secretion.(ABSTRACT TRUNCATED AT 250 WORDS)