Aspermatogenic seminiferous tubules were obtained from adult Wistar rats treated with Busulfan at the 20th day of foetel life. The isolated tubules converted 14C testosterone (T) into androstenedione (delta4), 3,5%) 5 alpha-dihydrotestosterone (DHT), (1%) ANd 5 alpha-androstane-3 beta, 17 beta-diol at unit gravity and incubated with 14C-T. Only delta was produced (2%). Using a similar technique the same result was obtained when pure preparation of round spermatids and primary spermatocytes from ram testis were incubated with 14C-T. From these experiments, we conclude that 5 alpha-reductase activity is present only in Sertoli cells.