Transforming growth factor-β1 (TGF-β1), which is induced in the prostate following castration, has been speculated to mediate apoptosis of epithelial cells during prostatic involution. Here, we report the first evidence of a direct effect of TGF-β on induction of apoptosis in prostatic epithelial cells in vitro, using NRP-152 nontumorigenic and NRP-154 tumorigenic rat prostatic epithelial cell lines. TGF-β1 induces apoptosis of both cell lines within 24 h, as shown by a decrease in cell viability, in situ DNA nick-end labeling, and internucleosomal DNA fragmentation. Moreover, the ability of TGF-β to induce apoptosis of NRP-152 is strictly dependent on culture conditions, because dexamethasone enhances while insulin and insulin-like growth factor-I specifically block apoptosis induced by TGF-β. We suggest that TGF-βs are direct physiological regulators of apoptosis of prostatic epithelial cells.