Despite important metabolic effects beta-blocking agents are usually well tolerated, but during metabolic stress important perturbations may occur. For instance, treatment with a non-selective beta-blocker leads to a slower rate of recovery from insulin-induced hypoglycaemia. 1 Similarly, blood glucose concentrations are more rapidly reduced during prolonged physical exercise during treatment with a beta-blocker.2 Serious hypoglycaemia has occurred during prolonged exercise in patients receiving treatment with propranolol3 and pindolol.4 We report on a patient treated with alprenolol, a non-selective beta-blocker with intrinsic sympathomimetic activity, who developed serious hypoglycaemia during endurance exercise.