The antiarrhythmic effects ofd-tubocurarine, gallamine and succinylcholine were evaluated in mongrel dogs mechanically ventilated with nitrous oxide-oxygen. Arrhythmias were induced by intravenous administration of epinephrine. The severity and incidence of ventricular arrhythmias were significantly reduced by the muscle relaxants in comparison with thiopental. Repeated injections of epinephrine were less effective in producing ventricular arrhythmias while they continued to produce the same degree of pressor response. The results suggest that neuromuscular blocking drugs may protect against some types of ventricular arrhythmias.