Blood levels of testosterone precursors, i.e. pregnenolone, progesterone, 17 alpha-hydroxyprogesterone, androstendione, DHEA, and delta 5-androstendiol as well as testosterone and estradiol are measured in 10 animals each of 10 different species. The determination is done by radioimmunoassay with steroidspecific antibodies. Precursors of the delta 5-pathway (DHEA, androstendiol) are low in the red deer, dog, cat, rat and guinea pig. Precursors of the delta 4-pathway (progesterone, 17-hydroxprogesterone, androstendione) are lower in the bull, boar, ram, stallion and rabbit thus indicating a predominance of different pathways in the animal species studied herein. Pregnenolone concentrations are of equal height in all animals, testosterone is lowest in the cat and stallion. In the latter species the estradiol/testosterone ratio is spectacular high.