A 23-year-old patient with a complete form of testicular feminization syndrome (Tfm) was examined endocrinologically and morphologically by electron microscopy. 1. Serum testosterone, DHEA and estrone concentrations and the urine 17-KS concentration were higher than in adult males. Furthermore, serum pituitary gonadotropins, especially LH, were increased. 2. We found that testicular venous testosterone, estrone, estradiol and progesterone were remarkably increased compared with their peripheral concentrations, and that circulating concentrations decreased rapidly after gonadectomy. The electron microscopic study showed active steroidogenesis in Leydig cells. We believe that these results prove that these steroids are produced in Leydig cells. 3. We found that the administration of 10,000 IU/day of hCG for 2 days increased serum gonadal steroids but that of 225 IU/day of hMG for 2 days did not, that LH responded strongly to LH-RH but FSH response was weak, and that after gonadectomy serum LH immediately increased but FSH did not change for at least 24 hours. From these results, we assume that FSH is not related to steroidogenesis and that the mechanisms regulating serum LH and FSH are different. 4. The seminiferous tubules were occupied by two kinds of cells. Both kinds were Sertoli cells, one of which was numerous and of low electron density indicating an immature type, and the other was of high electron density indicating a mature type. Spermatogonia were not detected.