Serum reproductive hormone levels were measured serially after eugonadal and hypogonadal men had received either a 200-mg or a 100-mg intramuscular injection of testosterone enanthate. The calculated mean integrated testosterone and estradiol levels indicated that the 200-mg testosterone enanthate injection in the hypogonadal subjects maintained eugonadal levels of these hormones through day 11. The 100-mg dose maintained eugonadal levels of these hormones through day 11. The 100-mg dose maintained eugonadal testosterone levels through day 7. The testosterone:estradiol ratios in both groups following the 200-mg injection remained above or at the eugonadal baseline trough day 21. The authors recommend that replacement therapy for hypogonadal men should be 200 mg of testosterone enanthate every 10 to 14 days. A similar dosage would be recommended if testosterone enanthate were to be used as an experimental inhibitor of spermatogenesis (contraceptive agent).