The androgen dependence of a highly abundant mRNA found in the rat dorsolateral prostate and seminal vesicles has been investigated using a complementary DNA clone from a rat dorsal prostate library. The 1.5 kilobase (kb) mRNA codes for a 52 000 Da translation product which is processed to 49 000 Da in the presence of microsomal membranes. This product appears to correspond to the previously described SVS II protein secreted by rat seminal vesicles and can be immunoprecipitated with anti-SVS II antiserum. Dot hybridization assays indicated that the mRNA is abundant in the dorsal and lateral prostate glands and in seminal vesicles but not in the ventral prostate, coagulating gland or other non-accessory sex tissues. Castration of mature male rats reduces the 1.5 kb mRNA 10-fold in the seminal vesicles and 7-fold in the dorsolateral prostate in 9 days. Androgen administration to one-week castrates returned the mRNA level to normal in both tissues within 48 h. The levels of the 1.5 kb mRNA are very similar in the dorsolateral prostate and seminal vesicles at maturity but distinct patterns of developmental regulation of this gene exist in the two tissues. Between 3 and 6 weeks of age, the level of the 1.5 kb mRNA increases approximately 3-fold in the dorsolateral prostate while the increase in the seminal vesicles is more than 600-fold.