Concentrations of glycogen, glucose, glucose-6-phosphate and lactate in the lateral vastus muscle were measured in seven subjects before and after dynamic muscle exercise at a work load of 75% of each subject's maximal working capacity, and with and without intravenous administration of the beta 1-selective beta-adrenoceptor blocking agent, atenolol. Pulmonary oxygen uptake was measured during exercise. Heart rate and arterial blood pressure were measured throughout the study. Arterial concentrations of glucose, lactate and free fatty acids were measured at rest and during exercise. The muscle concentration of glycogen and the extent of glycogen depletion with exercise were not influenced by the beta 1-adrenoceptor blocker. Similarly, there was no change in the muscle concentrations of glucose, glucose-6-phosphate and lactate. Heart rate decreased at rest and during exercise. Arterial blood pressure was not influenced by beta-blockade. Pulmonary oxygen uptake decreased by 6.5%. The exercise induced rise in arterial blood concentration of free fatty acids was abolished by beta 1-selective beta-blockade. It is concluded that the decrease in lactate release from exercising muscles during beta 1-adrenoceptor blockade seen in other studies cannot be explained by an impaired breakdown of muscle glycogen. It may be inferred, however, that a reduced availability of free fatty acids in the exercising muscles during beta 1-selective (and non-selective) beta-blockade may enhance the combustion of pyruvic acid and thereby decrease the production of lactate.