Serum FSH, LH, and testosterone were studied in 114 infertile men with poor sperm production. Testicular biopsies were taken and classified morphologically. In 90 specimens, the pattern of conversion of progesterone was determined in vitro and expressed as the ratio of 20 alpha-hydroxy-4-pregnene-3-one to 17 alpha-hydroxyprogesterone. An excess of the former metabolite indicates a prepubertal type of steroid metabolism. The normal limit of this ratio is given. The collected data indicate that the prepubertal type of steroid metabolic pattern is related to the thickness of the lamina propria of the seminiferous tubules only, and not to peripheral hormone levels. In particular, the presence of hyaline deposits in the lamina propria seems to determine the metabolic pattern. It is suggested that the character of the lamina propria separating the tubular and interstitial compartments of the testis is of crucial importance for the functional interrelationship between these compartments. This supports the concept of an intratesticular regulatory mechanism of both steroid metabolism and spermatogenesis.