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1. Conscious lambs less than 2 days old were exposed to thermoneutral and to cold environments, and the radioactive microsphere technique was used to obtain measurements of blood flow to most tissues of the body.

2. Cold stress was such as to evoke a summit metabolic response. Deep body temperature was within the normal range for thermoneutral conditions, while metabolic rate increased by about 300%, cardiac output by about 30%, and blood pressure by about 25%.

3. Blood flow to the thermogenic tissues increased: perirenal, inguinal, pericardial, prescapular and cervical brown fat depots showed a five- to sixfold increase, while the longissimus dorsi, trapezius, gastrocnemius and biceps femoris muscles showed a five- to ninefold increase.

4. There was an increase in blood flow in the diaphragm, intercostal, panniculus and cardiac muscles, but a decrease in flow in the skin of the leg, ear and midside, the maxillo turbinals, thyroids, spleen, kidneys, liver and gut. There were no significant changes in total or regional blood flow in the brain or spinal cord, or in the adrenals, nasal mucosa or tongue.

5. Results support other evidence that thermogenesis in both brown fat and skeletal muscle is very important in the new-born lamb exposed to cold. Changes in flow in other tissues correlated with known functions in the cold animal. The redistribution of cardiac output during cold stress is a further example of circulatory adjustments which enable animals to cope with stress states.

6. Many differences between blood flow values obtained by the radio-active microsphere and other techniques for measuring blood flow are noted and discussed.