In order to detect any possible Leydig cell dysfunction associated with male infertility, the endocrine capacity of the tests was investigated in vivo and in vitro in 21 infertile men. Plasma testosterone was determined before and after 3 days of hCG stimulation. Testicular tissue obtained by bilateral biopsies was subjected to (1) histological examination, (2) determination of basal testosterone concentration and (3) incubation with hCG. Patients were grouped according to histology. In vitro basal and stimulated testicular testosterone was similar in patients with normal histology, Sertoli-cell-only syndrome and spermatogenic arrest. Tissue from patients with Leydig cell hyperplasia showed 3-fold higher basal testosterone levels and a greater response to hCG. All patients had plasma testosterone levels and responses to hCG in the normal range. There was no significant correlation between the data obtained in vivo and in vitro, indicating that testosterone determinations in peripheral blood do not necessarily reflect the intratesticular situation. There was no evidence for gross abnormality in Leydig cell function accompanying disturbed spermatogenesis.