Rat pups, 10-12 days old, survived maternal deprivation if kept warm at 35 degrees, but died within 6 days if allowed to become hypothermic at a room temperature of 23 degrees. Normal body temperatures facilitated feeding, but even without food, warm pups survived starvation longer than cool ones. Increased survival could not be attributed to decreased oxygen consumption, and warm pups lost more body water and solids than cool pups. Striking differences in development that may have affected survival were observed over 72 hr of separation without food. Cool pups showed a virtual arrest of the growth of the tail, fur, tibia, and femur, and failed to increase brain weight, protein, DNA, RNA, and catecholamine contents. Warm pups showed developmental growth more comparable to that of normally mothered pups and significantly exceeded controls in the rate of accumulation of brain norepinephrine and dopamine.