The effects of estrogens on the growth and enzyme activities for androgen synthesis in a mouse Leydig cell tumor line (T 124958-R) were studied. The s.c. implantation of a diethylstilbestrol pellet resulted in a marked enhancement of the tumor growth. 5 alpha-Reductase activity (nmol/g/h) in tumors rapidly grown in the presence of diethylstilbestrol pellet was 4 times higher than that in tumors slowly grown in the absence of diethylstilbestrol, whereas an inverse relation was found for 17 beta-hydroxysteroid oxidoreductase activity. 17-Hydroxylase activities were similar in both tumors. The major C21- and C19-steroids formed from progesterone by the tumors grown in the presence of estrogen were 5 alpha-steroids such as 3 alpha- or 3 beta-hydroxy-5 alpha-pregnan-20-one, 3 alpha, 17-dihydroxy-5 alpha-pregnan-20-one, androsterone, and 5 alpha-androstane-3 alpha, 17 beta-diol, whereas the major steroids formed by the tumors in the absence of estrogen were 4-ene-3-ketosteroids such as 20 alpha-hydroxy-4-pregnen-3-one, 17-hydroxy-4-pregnene-3,20-dione, and testosterone. Furthermore, 10(-8) M of 17 beta-estradiol added in serum-free medium for 10 days significantly enhanced 5 alpha-reductase activities per 10(6) cells but significantly inhibited 17 beta-hydroxysteroid oxidoreductase activity in primary cell culture. These results indicate that estrogens stimulate the growth of T 124958-R in vivo and that estrogens may directly enhance 5 alpha-reductase activity but inhibit 17 beta-hydroxysteroid oxidoreductase activity in T 124958-R cells.