Inhibition of adipose tissue lipolysis may be involved in the impairment of endurance capacity after administration of a beta-adrenoceptor blocker. During endurance exercise, no significant decrease in plasma glycerol and free fatty acid (NEFA) concentrations after beta-adrenoceptor blockade is found. However, the levels during recovery from exhaustion are lower after beta-adrenoceptor blockade. This study was designed to investigate whether the lower levels after exercise are due to beta-adrenoceptor blockade or to the shorter time to exhaustion after administration of a beta-adrenoceptor blocker. In a single-blind study, 11 well-trained male subjects (age 23 (0.9) y) performed a cycle ergometer test at 70% Wmax until exhaustion 2 h after intake of 80 mg propranolol. One week later, the test was repeated after intake of placebo and was stopped at the time of exhaustion in the previous test. Average exercise time was 24 min. During exercise plasma glucose was lower, whereas plasma lactate and the respiratory exchange ratio were significantly higher when the subjects were on propranolol. Glycerol and NEFA concentrations during exercise were not significantly different between the two conditions. Despite an identical exercise time, glycerol and NEFA concentrations during recovery were significantly lower after propranolol treatment. In conclusion, lipolysis is inhibited during exercise after propranolol, probably causing a shift from fat to carbohydrate combustion.