The direct effect of estradiol on testicular androgen biosynthesis was studied in the frog, Rana esculenta, measuring progesterone, 17 alpha-OH-progesterone, androstenedione, and androgens (T + DHT) in supernatants and testes incubated (6 hr, 15 degrees) with estradiol alone (10(-6) M) or in combination with crude pituitary homogenate (1 pituitary equivalent/tube). Estradiol, either alone or in combination with pituitary homogenate, induced decreases of 17 alpha-OH-progesterone, androstenedione, and androgens but was ineffective in modulating progesterone levels. Pituitary homogenate was effective in inducing a significant increase of androstenedione and androgens but was ineffective in modulating both progesterone and 17 alpha-OH-progesterone production. It is concluded that estradiol acts by decreasing the activity of steroidogenic enzymes starting from 17 alpha-hydroxylase, while pituitary homogenate does not affect the 17 alpha-hydroxylase activity, but it acts starting from 17,20-lyase.