Human chorionic gonadotropin (hCG) induces profuse lobular development in the mammary gland of young virgin rats. To clarify whether the effect of hCG is locally mediated by inhibin, a non-steroidal glycoprotein, we detected its localization immunocytochemically in the mammary gland with polyclonal antibodies against the alpha- and beta-chains. Virgin female Sprague-Dawley rats were sacrificed after treatment with a daily IP injection of 100 IU hCG for 5, 10, 15, or 21 days of treatment or 20 days after the last injection. Whereas the mammary gland of control animals did not contain immunoreactive inhibin, hCG treatment induced the expression of inhibin in the cytoplasm of alveolar cells but not in ductal cells. The reaction became evident by Day 10 of treatment and reached its maximal intensity by Day 15. Thereafter, the reaction became evident in the stroma, which exhibited maximal positivity by Day 20. Once hCG treatment was terminated, the mammary gland regressed to its pre-treatment condition, appearing similar both in morphology and inhibin content to that of control animals. The expression of this glycoprotein hormone in the mammary gland after hCG administration at the time of maximal lobulolveolar development, and its diffusion towards the stroma during regression, suggest a critical role of inhibin as a modulator of mammary growth and differentiation.