Meiotic chromosomes and testicular histology were analysed in two postpuberal males with Down's syndrome. Both patients had the karyotype 47,XY,G+ without evidence of mosaicism. Complete spermatogenesis was demonstrated in both patients with normal or near normal testicular morphology. Meiosis was regular with the presence of two cytologically distinct classes of primary spermatocytes at diakinesis and metaphase I. Cells with 23 and 24 meiotic chromosome bodies were recognized in both patients. Trivalents could be neither identified nor excluded. Primary spermatocytes with 23 chromosome elements may either contain the supernumerary G‐chromosome in a hidden, e.g. trivalent, state or they may be truly normal due to previous elimination of the extra chromosome. In cells with 24 chromosomes a univalent G‐chromosome was tentatively identified.The clinically observed subfertility or sterility in mongoloid males is probably explained by a multitude of contributory factors, chief among which are an impaired postmeiotic maturation process and the general unfitness consistent with Down's syndr