Valle & Valle in 1943 demonstrated androgenic activity in the testes of Bothrops jararaca and Crotalus terrificus terrificus by bioassay. More recently Arvy (1962) has detected histochemically the presence of 3β-hydroxysteroid dehydrogenase in testicular interstitium of three species of reptiles. These data and reports of interstitial cell cycles correlated with reproductive periodicity in reptiles (Wilhoft & Quay, 1961; Wilhoft, 1963; Hahn, 1964; Wilhoft & Reiter, 1965) suggest the involvement of reptilian interstitial tissue in steroid synthesis. It was therefore decided to determine the steroid biosynthesis of reptilian testes in vitro. Six mature Natrix sipedon pictiventris weighing 188–330 g. were obtained from their natural habitat in Florida during September. Testicular weight ranged from 1·71 to 4·08 g.; 16·5 g. of minced testicular tissue was distributed between 18–50 ml. flasks so that 800 mg. tissue were contained in each. Incubation conditions were those previously described (Callard & Leathem, 1964). Each flask was