Ventral prostate glands from C3H mice of 4–6 weeks and 6 months age were grown in vitro by the watch-glass technique in the presence of testosterone propionate (t.p.) for periods of 10 and 21 days. Young glands grown in normal control medium show regression of the alveolar epithelium, but essentially retain their original histological structure if t.p. (50 μg/ml.) is added to the culture medium. Prolonged treatment is followed by hyperplasia of the alveolar epithelium. In older glands differentiation is maintained in normal control medium. Administration of t.p. induces hyperplasia of the alveolar epithelium with cytological and mitotic abnormalities shortly after treatment has begun, while prolonged exposure leads to degeneration of the alveolar epithelium. It is concluded that young glands need a higher level of androgens to preserve differentiation than older glands which may have acquired a greater degree of sensitivity to the hormones. The direct effects of t.p. are compared with those produced by oestrone and 20-methylcholanthrene and similarities observed in the action of the carcinogen and testosterone are discussed briefly.