1. Electron microscopic examinations revealed that most of the adipose tissue of new‐born calves had the cellular morphology of brown adipose tissue; only subcutaneous tissue had the cellular morphology of white adipose tissue. 2. The cellular morphology of the brown adipose tissue changed progressively to that of white adipose tissue as the age of the calves increased. 3. Infusion of noradrenaline (I.V.) at rates of 1 and 5 mug/kg.min into new‐born calves exposed to a thermoneutral environment increased metabolic rate two‐ to threefold, and also increased rectal temperature and respiration rate. The responses declined as age of calf increased. 4. Approximately 2% of the body weight of new‐born calves appears to be brown adipose tissue.