SUMMARY: The effect of hydrocortisone was studied in organ cultures of the ventral and anterior prostate gland of young rats grown in natural and semi-defined medium and the action of hydrocortisone on the ventral gland was compared with that of testosterone. In natural medium the ventral prostates showed a reduction of the number of alveoli, a lowering of the epithelium and a striking increase of the stroma. In the semi-defined medium the changes were confined to a lowering of the epithelium and a reduction of secretory activity. Hydrocortisone counteracted this regression in all cultures to which it had been added. In natural medium it preserved the number of alveoli, maintained epithelial height and suppressed the proliferation of the stroma. In the semi-defined medium the hormone preserved the height and function of the epithelium. Testosterone added to natural medium inhibited stromal growth and partially maintained the epithelium; in semi-defined medium it maintained the epithelium but the cells were better preserved by hydrocortisone. The anterior prostate gland showed epithelial hyperplasia and increase of connective tissue in both media; hydrocortisone inhibited both changes and produced cultures that resembled the organ in vivo.