In 30 isolated, spontaneously beating right atrial preparations of the rabbit, the arrhyth- mogenic actions of ouabain were studied with microelectrodes. Ouabain (10−6M) uniformly produced the following events: (1) a gradual increase in sinus rate with periods of alternation of beat-to-beat atrial intervals, (2) a rapid rate and regular rhythm, (3) severe irregularity of rhythm, (4) total atrial arrest. Cycle length decreased from 394 msec ± 43 (mean ± SD) during control to 229 msec ± 26 during the period of rapid rate and regular rhythm (P< 0.001). Pretreatment with atropine or propranolol did not abolish this positive chronotropic action of ouabain. Maps of the spread of activation in the area of the sinus node in eight experiments revealed that the speeding up of rate is accompanied by a shift of dominant pacemaker site toward the sinoatrial border. In explanation, it is shown that sinoatrial border fibers develop strong diastolic depolarization, whereas dominant pacemaker fibers do so to a lesser extent or not at all. The atrial arrhythmias-best to be described as "digitalis-induced sinus tachycardia"-may represent the experimental counterpart of the so-called "paroxysmal atrial tachycardia with block" described as a common manifestation of digitalis intoxication in man. Circ Res 46: 406-414, 1980