Dogs with indwelling venous and arterial catheters were trained to run on a treadmill (15%, 100 m/min) for 3-3.5 h. The rate of appearance of free fatty acid (RaFFA) and that of glucose (RaG) were measured simultaneously using [14C]palmitate and [3-3H]glucose as tracers (primed, constant rate infusion). In separate experiments [U-14C]glucose was used to estimate the rate of breakdown of extrahepatic (muscle) glycogen. Infusion of propranolol into the steadily exercising dogs caused a sharp decrease of RaFFA and a small transient increase of RaG. The ratio RaG/RaFFA rose from 0.4-0.5 to 1.5-1.7. Plasma lactate fell markedly due to the inhibition of muscle glycogenolysis. Instead of the usual 50%, now 80-85% of the lactate turnover originated from plasma glucose. The rising metabolic clearance rate of glucose could not be met by RaG and premature hypoglycemia ended the exercise. When propranolol infusion was started prior to exercise, blood glucose fell at a faster rate and plasma c[cyclic]AMP) rose at a significantly slower rate than in the controls. The breakdown of muscle glycogen rose in the first 5 min then fell to a level of about 15-20% of the rate measured under control conditions. During steady exercise, the mobilization of FFA and muscle glycogen is mediated by the .beta.-adrenergic system and is not related directly to the increased energy expenditure. The hepatic glucose output, although impaired by the .beta.-blockade, is also controlled by other factors (hypoglycemia-induced hyperglucagonemia, low insulin, .alpha.-adrenergic stimulation).