Serum levels of LH and FSH and the intratesticular concentrations of testosterone (T) and estradiol (E) were measured in biopsy tissue from 40 infertile men, aged 21-36 yr, of whom 21 were oligospermic men with varicocele and 19 were men with idiopathic oligospermia. Intratesticular T and E concentrations were negatively correlated (P less than 0.001) with testicular volumes, as measured with a calibrated orchidometer, suggesting that differences in measured intratesticular steroid levels in part reflect altered relative Leydig cell density as seminiferous tubular volume changes. To gather information about the regulation of intratesticular T and E by gonadotropins, we calculated an index of intratesticular steroid content by multiplying steroid concentration by testicular volume and compared these values with circulating LH and FSH levels. Highly significant positive correlations were found between both serum LH and FSH and intratesticular E content and between LH and FSH and intratesticular T content. Multivariant stepwise regression analysis revealed that while serum FSH is a strong predictor of intratesticular E (r = 0.72; P less than 0.001), serum LH is not (partial r = 0.00 when controlling for the influence of FSH). Instead, the apparent relationship between Serum LH and intratesticular E results from the highly positive correlation between serum LH and FSH in the patients studied (r = 0.71; P less than 0.001). Similarly, circulating LH levels are independently related to intratesticular T content (r = 0.67; P less than 0.001), whereas the relationship between FSH and T is indirect (partial r = 0.06 when controlling for the influence of LH). We believe that these associations suggest that the major regulator of intratesticular T content is LH and that FSH may be the important gonadotropin regulating intratesticular E.