Bull semen, according to Puntriano (1953), is androgenic, and epiandrosterone and aetiocholanolone have been reported in bull epididymal semen (Breuer, 1955). Seamark (1963), however, found no, or very little, testosterone or androstenedione in bull, boar or ram semen, nor was he able to detect dehydroepiandrosterone or aetiocholanolone. This paper presents the results of analyses of the semen and other fluids of the male reproductive tract using a double isotope dilution derivative procedure for estimating 'free' testosterone and dehydroepiandrosterone (Hudson, Coghlan, Dulmanis, Wintour & Ekkel, 1963). Semen was placed into dry ice on collection and stored deeply frozen until analysed. Testes and epididymides were collected from rams immediately after slaughter and packed in ice during transport to the laboratory. Testicular fluid was collected from these specimens after severing the outlet to the caput epididymidis or from living animals by cannulation (Voglmayr, Waites & Setchell, 1966). The contents of the caput were