The relationship between energy intake and body weight of reindeer and caribou are summarized. The results indicate that caloric intake was 35-45% lower in winter than during the summer growth period. The relation between heat production and body weight also exhibited a cyclical pattern. Heat production per unit of metabolic weight decreased by 25% (mid-August to mid-November). From calorimetry studies, the relative proportions of protein and fat deposition from weaning to 12 months of age were estimated. The amount of digestible nitrogen required for N equilibrium amounted to 0.462g N/W0.75 per day, a value comparable to those reported for cattle and sheep. The estimated metabolizable energy requirement for maintenance of a 70-kg reindeer in winter amounted to 5.5 Mcal/day, or about 200kcal/W0.75 per day.