A three-year-old, male Weimaraner weighing 46 kg was attended at the Veterinary Hospital of Vila Velha University with a main complaint of fatigue after exercise and a year before had an episode of weakness and syncope during strenuous exercise. Physical examination showed rosea mucous membranes, capillary refill time of two seconds, tachypnea, heart rate (HR) ranging from 140 to 180 beats per minute (bpm), systolic blood pressure 138 mm Hg, irregular rhythm and absence of heart murmur on auscultation. The electrocardiogram showed the presence of isolated premature ventricular complexes and paroxysmal tachycardia. Doppler echocardiography showed no morphological changes. Holter exam disclosed 1348 supraventricular extrasystoles in which 11 were in tachycardia and 8 paired. Sotalol hydrochloride was prescribed at a dose of 1.4 mg/kg every 12 hours. After 15 days from the beginning of the treatment, the animal was reevaluated presenting 60 to 80 bpm of HR with regularly irregular rhythm and, subjected to a new ambulatory electrocardiographic examination, presented only respiratory sinus arrhythmia with sinus arrest and first-degree atrioventricular block. The supraventricular arrhythmia in large dogs is generally associated with dilated cardiomyopathy; however, in this case, morphological changes were not observed. The sotalol hydrochloride is a nonselective β-blocker with antiarrhythmic class III effect. This drug may cause decrease in heart rate, reduction of supraventricular arrhythmia stimulus, and cause first-degree atrioventricular block, as could be observed in this case, in which only the use of β-blocker was able to control supraventricular arrhythmias.