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Two groups of normal male subjects were given an infusion of insulin and an infusion of epinephrine + insulin respectively. Blood glucose, plasma free fatty acids (FFA), insulin, growth hormone, cortisol, and urinary catecholamines were determined. Continuous indirect calorimetry was used to measure metabolic rate and oxidation rates of carbohydrate and lipids. The first group (n equals 7) received a 30-minute insulin infusion (0.1 IU/kg). While blood glucose and plasma FFA decreased, carbohydrate oxidation and metabolic rate significantly increased after some delay, whereas lipid oxidation decreased. The increase in carbohydrate oxidation amounted to 5 g/120 min. The decrease in blood glucose during insulin administration did not correlate with the increase in carbohydrate oxidation. In the second group (n equals 7), a 150-minute epinephrine infusion (900 mug in 500 ml saline) was administered, and superimposed upon it, a similar insulin infusion initiated after 60 min. Epinephrine alone increased blood glucose and plasma FFA levels, and decreased insulinemia. The rise in the metabolic rate was sharp and significant. After a short but significant increase the oxidation rate of carbohydrate decreased, whereas that of lipids markedly rose. This increase significantly correlated with that in FFA. Addition of insulin markedly decreased the elevated FFA levels and lowered blood glucose. After some delay this was followed by a marked increase in carbohydrate oxidation and a decrease in lipid oxidation. In this test the total increase in carbohydrate oxidation was 11 g/120 min. In comparison with the insulin test, this double amount seems to correlate well with the higher blood glucose levels measured before insulin administration. The results suggest that insulin indirectly stimulates carbohydrate oxidation by facilitating glucose transport into the cells and lowering FFA levels, and that epinephrine favours lipid oxidation through its lipolytic effects and its suppression of insulin release.