In an attempt to find suitable animal models to aid in the study of the reproductive processes of the human male, plasma levels of unconjugated pregnenolone, 17-hydroxypregnenolone, dehydroepiandrosterone, 20alpha-dihydroprogesterone, 17-hydroxyprogesterone, androstenedione, testosterone, dihydrotestosterone, oestrone and oestradiol were measured in 18 male baboons and 10 male rhesus monkeys and the steroid levels were compared with those previously established in normospermic, middle-aged men. Significant species differences were found with regard to the three delta5-steroids studied; whereas the approximate relationship of pregnenolone to 17-hydroxypregnenolone and dehydroepiandrosterone was 1:2:4 in men, the corresponding relationship was 1:5:30 in rhesus monkeys and 1:10:10 in baboons. Similar levels of 20alpha-dihydroprogesterone were found in the three species. On the other hand, the 17-hydroxyprogesterone levels in baboons were much lower and the levels of androstenedione lower than those found in men and in rhesus monkeys. No species difference was found with regard to circulating testosterone levels. However, both rhesus monkeys and baboons exhibited much higher levels of dihydrostestosterone than did men. Oestrone levels were higher in baboons than in men and oestradiol levels were higher in rhesus monkeys than in men and in baboons. The significant differences in circulating steroid levels suggest that further studies (including i.a. steroid analyses in testicular tissue, seminal plasma and spermatic artery and vein following both stimulation and suppression of testicular endocrine function) are required before preference can be given to any of the two species studied as a suitable animal model for the study of new fertility regulating agents.