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A 14monthold female beagle had ventricular preexcitation (VP).The finding was characterized by a wide positive QRS complex with a tall notched R wave in leads I, II, III, and aVF, an inverted QRS complex in leads aVR and aVL, a Q wave in lead I, and a short PR interval. Compression of the carotid sinus caused an anticipated marked decrease in heart rate, but did not reveal latent electrocardiographic abnormalities. We did not detect evidence of ventricular hypertrophy during echocardiography. Examination of the M-mode image indicated abnormal movements of the septum. There were 2 sharp waves at each systole instead of a single wider wave that is seen for clinically normal dogs. To further characterize this ECG finding, the affected dog and 2 clinically normal female beagles (positive control dogs) were given atropine (0.025 mg/kg of body weight, i. v.) Increases in heart rate, relative to values obtained before atropine administration, were evident in all 3 dogs. Increase in heart rate in the dog with VP appeared sooner after injection than in the clinically normal dogs; it was evident at the conclusion of the atropine injection. When the increase in heart rate was maximal in the affected dog (3 min after atropine administration), notching of the R wave disappeared, and the QRS duration decreased to about 60 ms. Echocardiographically, atropine produced a decrease in end diastolic, end systolic and stroke volumes in all treated dogs, which was similar between clinically normal dogs and the dog with VP. Atropine administration also was associated with a decrease in the percentage of thickening of the septum in the dog with VP, but not in the clinically normal dogs. We did not detect histopathologic abnormalities in the heart of the dog with VP.