In men with hypogonadotropic hypogonadism, prolonged treatment with LH and FSH induces spermatogenesis. To compare the respective role of exogenous testosterone and intratesticular testosterone on the induction and maintenance of spermatogenesis, 10 men with hypogonadotropic hypogonadism and without history of cryptorchidism were studied. They were treated with human gonadotropins (hMG; 150 IU FSH and LH and 1500 IU hCG, im, three times weekly) or pure FSH (150 IU, im, three times a week) and testosterone (T: 250 mg, im, once a week). Five men were treated first with hMG-hCG and then with pure FSH plus T. The other five men started with pure FSH plus T. Each treatment period lasted 24 months. In all men, hMG-hCG induced spermatogenesis after 24 months, with normal motility and quality. The combination of pure FSH and T was not able to induce spermatogenesis after 24 months. In addition, sperm count dropped dramatically to 0.3 +/- 0.1 x 10(6)/mL within 3 months and to 0 after 6 months when pure FSH and T followed [corrected] hMG-hCG. Plasma T levels were increased by both treatments, but significantly more after pure FSH and T (35.3 +/- 5.2 nmol/L) than after hMG-hCG (20.4 +/- 5.2 nmol/L; P < 0.05). Plasma estradiol levels after treatment with pure FSH and T were also increased, but the difference from those obtained during hMG-hCG treatment was not significant. In conclusion, in men with complete gonadotropin deficiency, FSH and exogenous T are not able to induce spermatogenesis. Furthermore, spermatogenesis induced by LH plus FSH (hMG-hCG) cannot be maintained when exogenous T replaced LH in the regimen. Thus, exogenous T is unable to replace LH (and intratesticular T) to induce spermatogenesis. These data are noteworthy in the prospect of male contraception after a complete blockade of gonadotropin activity.