The aim of the present study is to evaluate the histological and stereological changes in the right regressing ovary of newly-hatched chicks treated with follicle-stimulating hormone (FSH) during embryonic development. Stereological results indicate that FSH treatment in chicks diminishes the total volume of the right ovary and of lacunar channels, blood vessels, and interstitium; the number of germ cells also decreases. Results also indicate an increase in the total volume of interstitial cell cords in the right ovary of FSH-treated chicks. This increase is due to more abundant interstitial cells and not because of their cellular volume, which decreases in the FSH-treated right ovary. These findings suggest that the right ovary of newly-hatched chicks is able to respond to FSH, implying that FSH treatment favors an adequate microenvironment that induces structural changes and stimulates regressing of the right ovary.