We report here the influence of intratesticular aromatase activity and intratesticular estrogen concentration on spermatogenesis. In the present study, we measured the levels of aromatase activity and the concentrations of intratesticular testosterone (T) and estradiol (E2) in 21 testicular tissues from biopsy of 20 idiopathic male infertility patients, 5 testicular tissues from castration of 4 prostatic cancer patients and a testicular tissue from an autopsy. Serum T, Free T, E2, LH and FSH were also measured if possible. Aromatase activity in the testis were assayed by measuring the amount of 3H2O formed during the conversion of [1 beta-3H] androstenedione to estrogen. Histological evaluation of the testes were performed using the Johnsen's score count (JSC) method. The rate of aromatase activity was linear in regard to time and amount of tissue. The production of 3H2O was inhibited by 4-hydroxyandrostenedione. The apparent Km (Michaelis constant) of the reaction was 23.2 nM. The rate of aromatase activity increased linearly as JSC level decreased (r = 0.67). Also, it significantly correlated with intratesticular T (r = 0.69), E2 (r = 0.88) and T/E2 ratio (r = -0.85), whereas it was not correlated with serum T, free T, E2, T/E2 ratio and free T/E2 ratio. Therefore, our results suggest the possibility that the increase in the rate of aromatase activity and the concentration of E2 may influence spermatogenesis.