Ten men with idiopathic oligospermia were treated with delta 1-testolactone (Teslac), a potent inhibitor of conversion of androgens to estrogens. Teslac therapy caused a fall in serum estradiol and estrone levels of 34% (P less than 0.01) and 41% (P less than 0.01), respectively, and a rise in serum testosterone and androstenedione of 47% (P less than 0.02) and 70% (P less than 0.01), respectively. The testosterone-estradiol ratio increased by 126% (23.5 +/- 2.5 x 10(-1) to 45.2 +/- 5.0 x 10(-1); P less than 0.01) and the androstenedione:estrone ratio increased 231% (4.7 +/- 0.6 x 10(-1) to 12.9 +/- 2.0 x 10(-1); P less than 0.01). Basal and LRH-stimulated serum gonadotropin levels were unaffected by these changes. Sperm density rose from 10.8 +/- 2.5 to 19.8 +/- 4.7 x 10(6)/ml (P less than 0.01) and total sperm count from 26.8 +/- 6.5 to 60.6 +/- 14.3 x 10(6) (P less than 0.001). There was no significant change in motility or semen volume. Three of the wives became pregnant. There were no adverse effects of Teslac administration. These data suggest that Teslac may be an effective treatment for men with idiopathic oligospermia. Though the data do not clearly elucidate the mechanism of this effect, they are consistent with the possibility that Teslac's lowering of estrogen levels may have been responsible for the improvement in spermatogenesis. The findings also suggest that moderate increases in the circulating androgen-estrogen ratio do not affect gonadotropin output in men.