Mice of strains A2G and C57BL were kept in male-female pairs throughout their breeding life, (a) in an environmental temperature of -3° C, (b) at 21° C (controls). The mice at -3° C had an ancestry of several generations in the cold and had a much lower nestling mortality than mice recently introduced into the cold. In both strains, breeding began later and intervals between litters were longer in the cold than in the warm. The A2G mice produced and reared about the same number of young in the two temperatures; however, the breeding of the controls ended at about 40 weeks, while that of the mice in the cold was not completed until 80 weeks. The C57BL mice produced just over half as many young in the cold as in the warm: their breeding ended at about 52 weeks in both temperatures. Adrenal weights at 80 weeks were higher in the cold in C57BL mice but not in A2G. Kidney weights were higher in the cold in both strains, but especially C57BL. Spleen weights were lower in the cold in both strains; this may reflect a heavier infection with a pathogen in the control mice and is possibly relevant to the remarkably low nestling mortality in the cold.