Anaphylaxis to known allergens occurred in two patients under treatment for hypertension with propranolol. The clinical course of both cases was similar. Bradycardia associated with an undetectable blood pressure, unusual severity, and sluggish response to treatment were major common factors in which blockade of the beta-adrenergic system may have had a role. Propranolol, a beta-adrenergic antagonist that acts competitively by blocking the adenylate cyclase receptor on efferent cells, is well recognized to cause increased airways resistance in some asthmatic and normal subjects. It is postulated that propranolol potentiated anaphylaxis in these patients by inhibition of adenylate cyclase, resulting in lowered intracellular cyclic AMP and a lowered threshold of mediator release. The bradycardia during profound hypotension is attributed to an unopposed cholinergic action caused by blunting of the normal endogenous beta-adrenergic response by propranolol.