Meiotic studies were carried out on fertile male tertiary trisomic mice with the T(1;13)70H small translocation product, carrying the centromere of chromosome 1 and the telomere of chromosome 13 as the extra element. Approximately 200 primary spermatocytes from each of five males were studied. The only configurations found at diakinesis-metaphase I were 19 bivalents and a trivalent (22%) and 20 bivalents and an univalent (78 %). Among the cells with a trivalent, the majority (92.7%) appears to be of the type (13;13;113). This indicates that, in this case, the telomeric region of chromosome 13 has greater potential to form chiasmata than the proximal region of chromosome 1, which contains centric heterochromatin. From the presence of chromosome 113 in approximately 50 % (N=119) of the secondary spermatocytes, it can be inferred that the formation of an univalent in primary spermatocytes does not lead to loss of the extra chromosome at anaphase-telophase I. The impression was gained that the T70H small marker chromosome (113) can display positive heteropycnotic behavior in the tertiary trisomic males studied. Seven other T70H tertiary trisomic males were mated, generating 301 embryos and fetuses which were karyotyped at either 11 or 18 days of age. Of the 11-day-old embryos, 34.6 % contained the extra chromosome. Of the 18-day-old fetuses, this figure was 46.3 %. Gross differences in litter size of the tertiary trisomic mice occurred both within and between males. At day 12 of gestation, litter size (live embryos) averaged 4.44 ± 2.41 (2V=41). At day 19, the average number of live fetuses was 4.94 ± 2.75 (N=36). The low, albeit variable, reproductive performance of the tertiary trisomic males is caused mainly by lowered sperm production.