The effect of acute beta 1-blockade (100 mg metoprolol) and beta 1/2-blockade (2 mg levobunolol) on exercise performance and metabolism was studied in 16 healthy male physical education students. The study was carried out in a randomized double blind cross-over fashion. The maximal oxygen uptake was unchanged in all conditions, but maximal exercise capacity was reduced by 4% (beta 1-blockade) and 5% (beta 1/2-blockade). Maximal lactate concentration was reduced by 10% (beta 1-blockade) and 20% (beta 1/2-blockade). In 50 min treadmill exercise with 68% of maximal exercise capacity, beta 1/2-blockade led to premature exhaustion in 50% of subjects. Moreover, blood levels of glucose decreased (in some subjects to about 2.5 mmol X 1(-1], while the values were unchanged under beta 1-blockade. Free fatty acids and glycerol were similarly reduced by both beta-blocking agents. HGH, cortisol, adrenalin, and noradrenalin increased under beta-blockade, but more under beta 1/2-blockade than under beta 1-blockade. Insulin remained unchanged as compared with the unmedicated condition. The perceived exertion was elevated under both beta-blocking agents. CONCLUSIONS Beta 1-blockade affects exercise performance, especially endurance performance, less than beta 1/2-blockade due to unaffected beta 2-receptors mediating glycogenolysis in the skeletal muscles.