The gonadotropin responses to a 4-hour infusion of gonadotropin-releasing hormone (GnRH) and seminal plasma dihydrotestosterone (DHT) and testosterone levels were assessed before and 6 to 12 months after varicocelectomy in 22 men with varicoceles. Twelve men were severely oligozoospermic (sperm densities less than 10 X 10(6)/ml), whereas 10 men had sperm densities between 11 and 30 X 10(6)/ml. Each man had excessive gonadotropin responses to GnRH and lower than normal seminal plasma DHT levels preoperatively. Eight of the 12 severely oligozoospermic men and 6 of the other 10 men had postoperative improvements in sperm density. These men (responders) had normalization of gonadotropin response and seminal plasma DHT levels. The nonresponders had identical hormonal parameters before and after surgery. These results indicate that the pantesticular defect in hormonal synthesis and spermatogenesis, seen in some men with varicoceles, can be reversible.