The influence of verapamil infusion on the cardiovascular responses to lateral hypothalamic stimulation was studied in mongrel dogs anesthetized with chloralose. In both normal dogs and dogs with previous (20–30 h) coronary occlusion, it was observed that hypothalamic stimulation (1 mA, 2 ms, 100 c.p.s., for 10 s) induced an initial increase in coronary flow followed by a subsequent decrease. These changes were associated with increases in blood pressure, heart rate, and contractile force and pronounced arrhythmias (multifocal ectopic beats). Infusion of verapamil (18 μg/kg per min for 15 or 30 min) produced a transient increase in coronary flow and heart rate with a slight depression of contractile force, and antagonism of all arrhythmias in dogs with coronary ligation. After verapamil infusion, hypothalamic stimulation produced only increases in coronary flow, blood pressure, heart rate, and contractile force, with no associated arrhythmias. It is suggested that verapamil can antagonize experimentally produced arrhythmias by a combination of coronary vasodilator and cardiac alpha adrenergic blocking actions, rather than by a nonspecific 'quinidine-like' effect.