Rapid, clinical supraventricular tachycardia (SVT), without apparent underlying heart disease, was identified in 3 young Labrador Retrievers and 1 Labrador Retriever-type dog. The electrocardiographic characteristics, identification of ventricular preexcitation in 2 dogs, age at onset, response to antiarrhythmic agents, and recent electrophysiologic documentation of a concealed accessory pathway in another young Labrador Retriever suggested orthodromic atrioventricular reciprocating tachycardia as the mechanism of SVT.