The simultaneous subcutaneous injections of dihydrotestosterone (0.5 mg/kg) and/or ovine prolactin (5 or 50 IU/kg) to mature castrate rats for periods of 5-10 days led to an augmentation of prostate gland levels of RNA, DNA and protein. Of the various lobes of the prostate gland, the dorsal portion seemed to be particularly sensitive to the effects of the combined hormone treatment. In general, the highest dose of prolactin (50 IU/kg) and the longer duration of administration (10 days) produced the most consistent augmentation responses between dihydrotestosterone and prolactin as compared to those rats receiving only androgen. These findings reveal that certain lobes of the prostate are more responsive to hormonal interactions, but do not clarify the mechanisms involved in causing such enhancements of prostate cell growth and proliferation.