Two different reciprocal translocations between chromosomes 1 and 13 in the mouse, T(1;13)70H and T(l;13)lWa, were intercrossed, resulting in T(l;13)70H/T(l;13)lWa animals, possessing two heteromorphic marker bivalents. The meiotic behavior of these animals was compared to that of T(1;13)70H/T(1;13)70H animals. In females the chiasma frequency in the translocation bivalents was decreased by the presence of the loops following meiotic pairing, but in males chiasma frequencies were not changed. The nondisjunction level of both translocation bivalents was increased in females and males. The nondisjunction level of bivalents not involved in the translocations was increased in the females, but not in the males. Possible causes and relevance of these phenomena are discussed.