Gene expression in the rat dorsolateral prostate gland has been studied using cloned cDNA probes to the most abundant expressed mRNAs. One cDNA clone (pM-40) corresponds to two closely homologous mRNAs of about 880 nucleotides which code for two proteins of 23 and 21 kilodaltons (kDa). At least the 23-kDa protein contains a signal peptide. Another clone (pRWB) corresponds to a 1550-nucleotide mRNA which codes for a 52-kDa protein which also contains a signal peptide. The steady-state levels of these specific mRNAs increase in the dorsolateral prostate with sexual maturation. In castrated mature male rats, the M-40 mRNAs are inducible either by androgens or zinc, while the RWB mRNA is only responsive to androgens. In situ cDNA-mRNA hybridization histochemistry has been used to study the localization of the M-40 and RWB gene transcripts. Both M-40 and RWB mRNAs are most abundant in the epithelium of the lateral tip of the dorsolateral prostate. Following castration, the RWB mRNA decreases, while the M-40 mRNAs continue to be expressed in isolated areas of the epithelium. These castration-resistant cells maintain normal morphology in the absence of androgens.