Seven lean healthy young men were studied for 6 weeks during exposure to pharmacologic inhibition or stimulation of the sympathetic nervous system. For a period of 2 weeks their β-adrenergic receptors were either blocked with propranolol hydrochloride (160 mg/d) or stimulated with terbutaline sulphate (15 mg/d). After a further 2 weeks of placebo administration (500 mg lactose/d), the subjects crossed over to the drug they had not been taking at the beginning of the experiment for another 14 days. During the last five days of each 2-week period, the subjects consumed a weight-maintaining diet, composed of 12% protein, 48% carbohydrate, and 40% fat. They consumed exactly the same menus on the same days during the subsequent study periods. Body weight and physical activity were measured every day for 6 weeks. Daily heart rate and nitrogen excretion were measured continuously for four days at the end of each 2-week period, the last two days of which were spent in a respiration chamber where energy expenditure and a variety of metabolic parameters were measured. In the respiration chamber on the propranolol, placebo, and terbutaline treatments, respectively, significant differences were observed in mean daily heart rate (65 ± 3,75 ± 4, and 84 ± 4 beats/min), mean sleeping heart rate (51 ± 2, 56 ± 3, and 62 ± 3 beats/min), nitrogen excretion (13.6 ± 0.7, 12.6 ± 0.6, and 11.9 ± 0.6 g/d), fat oxidation (+1,045 ± 95, +1,243 ± 148, and +1,278 ± 84 kcal/d) and thyroid hormones (12.0 ± 0.7, 15.7 ± 0.9, and 17.2 ± 1.0 T 3/T 4 ratio). Although a 24-hour energy expenditure tended to be lower with propranolol (2,820 ± 187 kcal/d) than with the placebo (2,884 ± 174 kcal/d) and terbutaline (2,880 ± 122 kcal/d) treatments, the difference was not significant. The results are discussed in relationship with the possible use of β-adrenergic receptor agonists in the treatment of obesity.