In 3 adult males with trisomy-G, Down's syndrome, the chromosomes were studied in spermatogonial mitoses and in the first meiotic division. The findings were similar in all cases and are here presented together. Of 8 spermatogonial mitoses of good quality, 4 had 46 chromosomes and 4 had 47 chromosomes. At diakinesis-metaphase I, 21% of the cells had 1 G-bivalent and 1 trivalent, 38% had 2 bivalents and 1 G-univalent and in 41% 2 chromosomal elements composed of G-chromosomes were seen, but it was not possible to determine unequivocally whether 1 of them was a trivalent or not. From their morphology and from the spermatogonial chromosome counts it was tentatively concluded that at least some of them had only 2 G-bivalents. Premeiotic elimination of 1 G-group chromosome is a possible explanation of this phenomenon. The study of larger samples of spermatogonial mitoses should allow definitive conclusions to be made. — Different trivalent and univalent configurations were described. — Spermatogenesis was quantitatively assessed and found to be complete but of lesser magnitude than in a normal male. Spermatogenetic arrest was not noticed. As judged from histological sections of the testes, all 3 G-trisomic males would have to be considered fertile.