Basal concentrations of FSH, LH, testosterone, delta4 androstenedione and dihydrotestosterone, together with FSH and LH responses to single injections of LHRH were determined in eighty-four patients with oligospermia and in twenty-seven normal men. LHRH responses were heterogeneous and indicate that various disorders might cause this syndrome. In six cases there appeared to be an isolated deficiency in spermatogenesis, as indicated by an increased FSH response, whilst the LH response was normal as were the concentrations of the testicular hormones. In twenty cases a concomitant disorder of Leydig cell function and spermatogenesis is suggested as indicated by increased FSH and LH responses and decreased concentrations of testosterone and delta4 androstenedione (six) or concentrations at the lower limit of normal (fourteen). Furthermore, in five cases a hypothalamic and/or pituitary disturbance may be accepted on the basis of normal or decreased basal concentrations decreased and responses to LHRH with decreased concentrations of testosterone and delta4 androstenedione. Finally, in thirty-seven cases, oligospermia was not associated with any modification basal gonadotrophin concentrations or response to LHRH when compared with normal subjects.