Various studies have led to our present knowledge of α1-adrenoreceptors and their role in the regulation of cardiovascular physiology. Our study aimed to study the effect of the α1-adrenergic receptor agonist methoxamine on the frequency of spontaneous activity and electrical activity parameters of the atrial myocardium with a preserved sinus node and an imposed rhythm in newborn rats. In working cardiomyocytes of newborn rats with a preserved sinus node, methoxamine caused an increase in the frequency of spontaneous activity. α1-adrenergic receptor agonist methoxamine increased the duration of the repolarization phase of the action potential in both the imposed and the own rhythm.