In this study, we report the cloning and sequencing of a full-length cytokeratin-8 (CK-8) cDNA from a rat prostatic epithelial cDNA library. The effects of androgen on CK-8 expression in rat prostate were also studied. The data indicated that the steady-state level of CK-8 mRNA was elevated by about 5- to 10-fold in both prostate and seminal vesicles in castrated rats, and the elevated levels persisted during a 23-day experimental period. Androgen but not estrogen administration repressed the expression of CK-8 mRNA. This effect could be antagonized by the simultaneous administration of an antiandrogen, flutamide, indicating that CK-8 is a new class of androgen-repressed genes whose regulation is presumably mediated by androgen receptor mechanisms.