Sertoli cells from rats 6-60 days of age were examined for their in vitro capacity to convert androstenedione (A) to testosterone (T), androstenedione to 3.alpha., 17.beta.-dihydroxy-5.alpha.-androstane (DIOL) and 17.beta.-hydroxy-5.alpha.-androstan-3-one (DHT) to DIOL. The influence of FSH [follicle stimulating hormone] on these conversions was also examined. The A to T conversion rate, indicative of 17.beta.-oxidoreductase (17.beta.-HSD) activity, increased from less than 10 ng/h per mg in cells from 6 day old rats to over 60 ng/h per mg in cells from 16 day old rats and then declined to about the 6 day level in 32 day old rats; the rate of 17.beta.-HSD activity remained low in Sertoli cells from rats older than 32 days. FSH resulted in significant increases in A to T conversions in Sertoli cells from 10, 16, and 20 day old rats but not in cells from younger or older rats. The conversion of A to T was highly favored over the reverse reaction; the T to A conversion rate was not influenced by FSH. The 17.beta.-HSD activity showed a response to purified (Papkoff) FSH which was dose dependent between 1-100 ng/ml medium. The conversion rates of A to DIOL and DHT to DIOL, indicative of 5.alpha.-reductase and 3.alpha.-HSD activities, respectively, varied with age in a manner similar to the 17.beta.-HSD activity. However, FSH caused a significant increase in 5.alpha.-reductase and 3.alpha.-HSD activities only in cells from 10 day old rats. FSH caused a significant reduction of the enzyme activities in Sertoli cells from rats 20 to 24 days old. Recrystallization data indicated an absence of 3.beta.-HSD activity in Sertoli cells from young rats; no 3.beta.-androstanediol was found. Apparently, at the onset of meiosis, Sertoli cells possess enhanced 17.beta.-HSD, 5.alpha.-reductase and 3.alpha.-HSD activities which may be responsive to FSH. The FSH response is not present in Sertoli cells from older rats.