Because the mechanism of adverse reactions to abrupt cessation of propranolol in patients with coronary heart disease was an enigma, we studied the effect of cessation of propranolol on beta-adrenergic receptor reactivity to catecholamine stimulation. The cyclic AMP concentrations in plasma and left ventricular muscle after the administration of isoproterenol (5 mg/Kg) were measured in rats before, during, 2 days after, and 4 days after of the administration of propranolol (5 mg/Kg). Two days after withdrawal from propranolol, the cyclic AMP concentrations in plasma and left ventricular muscle were significantly increased (p less than 0.005, p less than 0.01). Four days after withdrawal from propranolol, the cyclic AMP concentration in plasma was significantly increased (p less than 0.001). On the other hand, that of left ventricular muscle showed a tendency to have higher value, although, this was statistically not significant. From these results, this study suports that there is a hypersensitivity to adrenergic stimultion after abrupt cessation of long-term propranolol treatment. The explanation of propranolol withdrawal phenomenon most likely lies in lthe nature of beta adrenergic receptors that become activated during long-term blockade.