Three classes of highly inbred 16-week-old mice, of strain A2G/Tb, were studied: (i) controls, kept at 21 degrees C; (ii) 'new stock', of the first or second generations reared in a room kept at -3 degrees C; (iii) 'old stock', of the fourteenth generation reared at -3 degrees C. All were bred at the same time. The new-stock mice were lighter than the controls, their tails were shorter, and they (especially the females) had less fat. They contained more water, and less nitrogen and collagen. The males, but not the females, had less calcium and phosphorus. The heart, stomach and small intestine were heavier than in the controls, and the intestine was longer. Liver and kidneys, too, were heavier, but these differences were large only in the females. The shaved skin was lighter, but the hair, especially in the females, heavier. Gonad weights were lower. The spleen tended to be lighter, but varied greatly. Most of these differences from the controls resemble those seen in adult laboratory rats exposed to cold for a few weeks. The old stock, of both sexes, had almost the same body-weight as the controls, but resembled the new stock in tail-length: hence the effect of cold on growth of the tail was independent of body-weight. The old-stock males had more fat than the controls, while the females were intermediate between the controls and the new stock. The males contained less water than the controls; while in nitrogen, collagen, calcium and phosphorus they did not differ significantly. The tendency to return towards the control state was less marked in the females. In weights of stomach, intestine and liver the old-stock males again resembled the controls; and their kidneys were lighter. They had, however, longer intestines than the controls and, like the new stock, they had heavier hearts. Their skin was lighter but hair, heavier. The spleen was lighter. Except in their kidneys and spleen, the old-stock females resembled the new-stock females, rather than the controls. Since other work has shown that the old-stock females are more efficient mothers than the new-stock, the old-stock mice of both sexes were clearly better adapted to cold than the new. This may have been due to intracellular changes making individual tissues more efficient. The change in the old stock cannot be due to selection of favourable genotypes; it may be due to a cumulative maternal effect.