1) Cardiac arrhythmias were produced in 8 mongrel dogs (weight, 13 3±1.5 kg) by injections of epinephrine (0.002±0.0002mg/kg) during inhalation of 25 per cent cyclopropane in oxygen. 2) Femoral arterial pressure was measured with a Statham transducer, and Lead II of the electrocardiogram recorded on a Sanborn Twin-Viso at a paper speed of 25 mm/sec. 3) Regitine (0.47±0.19 mg/kg), an alpha adrenergic blocking agent, significantly decreased the pressor response to epinephrine and consistently increased (4.8±1.8 times) the individual threshold doses of epinephrine required to produce cardiac arrhythmias. 4) In most instances the protective result was attributable to a modification by Regitine of the pressor effect of epinephrine. It is concluded in the light that cyclopropane-epinephrine cardiac arrhythmias are blocked by alpha adrenergic blockade.