The purpose of this investigation was to study heat production in the newborn rat, and the factors which may be limiting maximum heat production in the cold. Rats of varying ages between 1 and 30 days were exposed to various environmental temperatures. The minimum oxygen consumption in the warm, and maximum oxygen consumption in the cold, increased with age. The critical environmental temperature, and the environmental temperature at which oxygen consumption was maximal, fell with increase in age. Exogenous noradrenaline raised the oxygen consumption of 6-day-old but not of 12-day-old rats above the maximum oxygen consumption found with cold exposure. The cardiac output and arteriovenous oxygen difference of 12-day-old rats was increased in the cold. These results are consistent with the view that oxygen consumption in the cold is limited in 6-day-old rats by the release of endogenous noradrenaline and in 12-day-old rats by cardiac output and the supply of oxygen to the heat-producing tissues.