The effect on spermatogenesis of the implantation of six rods containing levonorgestrel and three estrone rods was studied in seven volunteers aged 32-40, with normal pre-treatment spermiogram. None of the subjects achieved azoospermia. The lowest sperm count was below 1 million/ml in three subjects and below 12 million/ml in the other four. Plasma FSH and testosterone were severely depressed during the study, but LH was only transiently depressed for the first two months. Four subjects recovered pre-treatment sperm counts in spite of the FSH levels below 1.5 mIU/ml and testosterone levels below 1 ng/ml, which is contradictory to the current concept on hormonal regulation of spermatogenesis.