A previous survey of 48 subjects with XYY chromosome complement showed a wide scatter of values for plasma concentration of testosterone and luteinizing hormone (LH). To account for this scattering and in view of the impulsive behavior often attributed to XYY subjects, it was postulated that paroxysmal activity in the cerebral cortex produced paroxysmal stimulation at the level of the hypothalamus. To investigate this possibility, plasma concentrations of testosterone, dihydrotestosterone, follicle stimulating hormone (FSH), and LH were determined on seven consecutive days in four XYY subjects and five XY control men as well as bimonthly for four months in two XYY detainees and three XY control detainees. The variability of plasma androgen and gonadotropin levels in XYY subjects was similar to that of XY control men. The results thus do not support the above hypothesis as an explanation of the widely scattered plasma androgen values in XYY individuals. The "XYY syndrome" is probably heterogeneous and includes a number of patients with some degree of hypogonadism. Testicular biopsy, sperm count, and meiotic studies were carried out in eight XYY men. In one case the meiotic study showed two Y-chromatin bodies. Results of these various investigations support the diagnosis of maturation arrest of the germ cells with consequent oligospermia. Low fertility is therefore expected to be frequent among XYY subjects, although when fertilization does occur, it can result in normal XY or in XYY infants.