The effects of androgen on cytokeratin (CK) 8 mRNA expression in rat ventral prostate (VP) were studied by in situ hybridization with an antisense RNA-probe. It was found that 1) the CK 8 antisense RNA-probe was accumulated only within prostatic epithelial cells, indicating that the CK 8 mRNA is a specific and sensitive marker for prostate epithelia. 2) After castration, the signals of CK 8 mRNA in VP sections were significantly increased. Administration of androgen to the castrated male host repressed the elevated expression of CK 8 mRNA in VP. This effect can be antagonized by the simultaneous administration of an antiandrogen. 3) Unlike other androgen-repressed gene, the elevated expression of CK 8 mRNA in VP was persisted even 2 months after the process of glandular involution was completed. 4) During prostate development, the strongest CK 8 mRNA staining was found in the early neonatal prostatic epithelia which were composed mainly of prostatic stem cells. Thereafter, when the levels of serum androgen were gradually increased, the shift of CK 8 mRNA staining to peripheral region and decreased level of overall CK 8 mRNA were noted. These data support the notion that the CK 8 gene of prostate epithelia is a new class of androgen-repressed one. The data also indicated that the expression of CK 8 gene is closely related with the proliferation and differentiation of prostate stem cells, and excessive expression of CK 8 mRNA is a characteristic for prostatic stem cells.