The effects of veratramine on transmembrane potentials of the isolated right atrial preparation of the cat were examined. Veratramine slowed spontaneous rate and precipitated a characteristic rhythm consisting of alternating periods of asystole and normal atrial rhythm (periodic rhythm). After large doses or continued exposure to low doses of veratramine electrical and mechanical activity of the atria ceased; subsequently activity resumed only in a discrete area within which action potentials were consistent with pacemaker characteristics. This discrete pacemaker area, easily located by sight, greatly facilitates study of the sino-atrial node. A gradation of veratramine effect in cells lying between the quiescent common atrial cells and the active pacemaker suggests the existence of "transitional" cells with electrophysiological responses intermediate between common atrial cells and pacemaker cells.
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