Homogenates of the epididymis, testis and seminal vesicle from 15, 25, 35 and 60-day old golden hamsters were incubated with [3H]4-androstene-3,17-dione and NADPH and enzyme activity was estimated. In the testis, activities of 5 alpha- and 5 beta-reductases were the highest (32 +/- 4 and 68 +/- 13 (SD) nmol/100 mg protein/h, respectively) at 25 days of age, moderately high at 35 days and very low (1-3 and 5-6, nmol/100 mg protein/h, respectively) at 15 and 60 days. In the epididymis, 5 alpha-reductase activities increased with age during sexual development by up to 50-fold and reached the maximum value (40 +/- 11 nmol/100 mg protein/h) at 60 days. The 5 alpha-reductase activities in the seminal vesicle were found to be relatively low (0.5-1 nmol/100 mg protein/h), compared with those in the epididymis and testis. Activities of 5 beta-reductase in the epididymis and seminal vesicle were very low (0.1-0.5 nmol/100 mg protein/h) at all ages. These results indicate the existence of a marked, but transient, increase in 5 alpha- and 5 beta-reductase activities during immature period in the testis of golden hamster. In the epididymis, however, 5 alpha-reductase activity increases with age during sexual maturation, while 5 beta-reductase activity is almost undetectable at all ages.