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Plasma epinephrine, norepinephrine and substrate concentrations (glycemia, free fatty acids (FFA), lactic acid) have been compared in same dogs exposed to cold then made to perform a running test. Under both conditions energy expenditure was increased to the same level for each dog, approximately 7 fold the metabolic rate at rest: 1. Catecholamines were increased in cold exposed as well as in running dogs. However plasma epinephrine and norepinephrine concentrations were respectively 2,5 and 3 times higher during cold exposure than they were during muscular exercise. 2. Under both conditions hyperglycemia was observed; however it was more noticeable during cold exposure than during running. Lactic acid increased under both conditions but average concentrations were not significantly different. FFA were increased only during cold exposure. 3. It may be concluded that for the the same level of energy expenditure a greater degree of involvement of the adrenal medulla and sympathetic nervous system is to be found during cold exposure than during exercise. This difference may partly explain that the relative contribution of substrates to the energy expenditure could be different during cold exposure and muscular exercise.