Sino-auricular block is a term applied to a relatively uncommon arrhythmia in which, in an otherwise regular rhythm, dropped beats occur, both auricular and ventricular beats participating in the intermittence. This definition is not strictly true because sino-auricular block is frequently associated with sinus arrhythmia. The long pause is not always equal to exactly two normal cycles, being sometimes shorter, at other times longer. Occasionally, too, the long cycle is preceded by a quickening of the auricular rate and is followed by cycles that are of somewhat greater duration than the average cycles of the normal rhythm. For a discussion of the literature on the subject of sino-auricular block, reference may be made to Lewis,1 and also to the papers of Levine2 and Eyster and Evans.3 Most of the articles are concerned with reports of clinical cases. In them the view has been generally accepted that the arrhythmia is due