1. The action of insulin on the net exchange of glucose, free fatty acids and glycerol by brown adipose tissue of young rabbits was investigated.2. Infusion of insulin (10,000 muu./kg.min I.V. for 10 min) caused a large increase in the uptake of glucose by brown adipose tissue of fed week-old rabbits. The release of fatty acids fell but glycerol release was unchanged.3. The brown adipose tissue of 9-day-old rabbits fasted in a warm environment had a high initial rate of fatty acid and glycerol release and low rates of glucose uptake. Insulin infusions (10,000 and 100 muu./kg.min) greatly reduced fatty acid release but had little or no effect on glucose uptake.4. The brown adipose tissue of 9-day-old rabbits fasted in a cold environment was depleted of fat and took up free fatty acid as well as glucose from the circulation. Infusion of insulin (10,000 muu./kg.min) caused a large increase in glucose uptake accompanied by a reduction in fatty acid uptake.5. The experiments support the view that insulin has a direct effect on fatty acid transport across the cell membrane.