Androgen biosynthesis in vitro by testes from (C57BL X WN)F1-WV/Wn (congenitally germ cell-free) and +/+ (normal) mice of different ages was investigated using [3H]progesterone as substrate. At all ages examined, the seminiferous tubules of the WV/Wn genotype were largely devoid of germ cells. However, the age dependent pattern of progesterone metabolism by germ cell-free testes were similar to that by normal testes. Conversion of progesterone to 5alpha-reduced 17-hydroxy-C21 and C19-steroids such as 3alpha,17alpha-dihydroxy-5alpha-pregnan-20-one, androsterone, 5alpha-androstane-3alpha,17beta-diol and 5alpha-androstane-3beta,17beta-diol was high in testes of 28- and 35-day-old mice of both groups. However, these 5alpha-reduced metabolities were found to be formed in very small amounts or not formed at all in testes of 20-, 120- and 150-day-old mice of both groups, in which conversion of progesterone to 17alpha-hydroxyprogesterone, androstenedione and testosterone was observed. It appears that in the absence of germinal cells, there is a normal increase and reduction of 5alpha-reductase activity in one or more classes of remaining cells during maturation of the testis.