Seven unconjugated steroids were measured in the blood and seminal plasmas of fertile male rabbits by radioimmunoassay. The blood plasma testosterone concentration was 4-5 times that of the seminal plasma. Dehydroepiandrosterone, estrone and 17 β-estradiol were found in measurable amounts in the blood plasma; however, these steroid levels were slightly lower in seminal plasma. Androstenedione and 5α-dihydrotestosterone were present in equal quantities in both the seminal and blood plasmas. By contrast, seminal plasma pregnenolone level was about twice that of the blood plasma. The determination of seminal plasma steroids may lend itself as a complementary assessment to blood steroid determinations for the evaluation of the normal function of various reproductive organs.