The effects of two beta-adrenergic blocking agents, pronethalol and propranolol, on ventricular fibrillation thresholds were tested in dogs. Pronethalol (4.0 mg/kg, 5 dogs) produced a mean increase of 45% in the fibrillation threshold. Propranolol (0.4 mg/kg, 7 dogs) failed to produce a significant change of the fibrillation threshold. To examine the possibility that the difference between the responses to pronethalol and propranolol might be related to the difference in dose of the beta-blocking agents, 4.0 mg/kg of propranolol was given to 3 dogs; this dose also failed to produce a consistent change of ventricular fibrillation threshold. These data indicate that under the conditions of our experiments it was possible to block the beta-adrenergic receptors with propranolol without producing a change of the ventricular fibrillation threshold. The increase of the fibrillation threshold produced by pronethalol was probably a consequence of its quinidine-like properties rather than of its beta-receptor blockade.