The urinary excretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) was measured by specific bioassays in 122 infertile men from barren couples in order to study the relationship between levels of FSH and LH and the state of the germinal epithelium. The latter was assessed both by determination of the gametic output and by examination of testicular histology, using the testicular biopsy score count method, in which the mean score (MS) is the overall measure of level of spermatogenesis (Johnsen 1970a). A highly significant negative correlation was established between log FSH and both log sperm concentration and MS and also between log LH and MS, but not between log LH and log sperm concentration. Of the 122 infertile men, the 56 who belonged to the heterogeneous idiopathic oligospermia group were studied most intensely. Nineteen of these had elevated urinary FSH levels. The mean FSH level for this group was 2.3 times higher than the mean FSH level for normal males (P < 0.001). Only 5 patients in this group had an elevated urinary LH level and the mean LH level was not different from that of normal men. In this idiopathic oligospermia group there was a significant negative correlation between log FSH and MS, and between log FSH and sperm concentration, but no correlation between log LH and either of the same 2 parameters. No correlation between log FSH and log LH on the one hand and the excretion of oestrogens, androgen metabolites, sperm morphology or motility on the other hand was found. The excretion of testicular and adrenal androgen metabolites (separated by a dexamethasone-suppression test) was normal in the 56 males with idiopathic oligospermia. It is concluded that there is a close relationship between urinary FSH levels, and to a lesser degree urinary LH levels, and spermatogenesis and that testicular histology is a better parameter for the judgment of the state of the germinal epithelium than the sperm concentration. It is further concluded that analyses of gonadotrophins, especially of FSH, are of value in the differential diagnosis of the infertile man, especially in distinguishing between those who will benefit from a treatment with gonadotrophins and those who will not.