Out of 27 cases of ventricular and supraventricular parasystole observed in [1969][1970] 7 cases of atrial, 4 cases of A-V junctional and 2 cases of sinus parasystole were presented.1. In 13 cases of supraventricular parasystole, exit block was found in 92 per cent, atrial fusion in 38 per cent, intermittent hi-and trigeminiy in 92 per cent, aberrant ventricular conduction in 82 per cent, resetting of the basic rhythm by the parasystolic beat in 75 per cent, and A-V prolongation of the parasystolic beat in all cases of atrial parasystole and in 1 of 3 cases of A-V junctional parasystole.2. Two cases of sinus parasystole revealed that atrial or A-V junctionaI escape rhythm can be induced not only by sinus slowing but also by sinus acceleration with intermittent, repetitive, second degree S-A block.3. The following ECC findings strongly indicate the presence of supraventricular parasystole: a) intermittent or alternating occurrence of bi-and trigeminy, b) concurrent occurrence of a premature beat with a compensatory pause and that with interpolation and a short returning cycle, c) various forms of atrial fusion waves, and d) various degrees of ventricular aberration with variable coupling intervals.4. In atrial and A-V junctional parasystoles, the retrograde A-S conduction pathway is less refractory than the antegrade A-V conduction pathway, which causes 2 kinds of exit block. One is a hi-directional exit block in which there occurs neither antegrade nor retrograde conduction. The other is a uni-directional exit block in which there occurs retrograde conduction which is not accompanied by antegrade conduction.Atrial parasystole was first reported by Kaulmann and Rotberger (1921), A-V junctional parasystole by Scherf and associates (1960) and ~inus parasystole by Schamroth (1967). Scherf and Schott (1953) defined pararrhythmia as an arrhythmia in which two centers concurrently and independently produce impulse, s which yield contractions of the whole or parts of the heart, without disturbances of conduction of the normal impulse being responsible for the arrhythmia. According to Schamroth (1964) who differentiated parasystole from dissociation with interference, parasystole is a dual rhythm whereby a) pacemakers concurrently and independently contribute to the rhythm of the heart, and b) the ectopic pacemaker is protected from the, impulses of the other pacemaker, such protection being situated within, or in the immediate vicinity of, the ectopic "I The outlines of this paper were read before the 36th annual meeting of the Japanese Circulation Society, Kanazawa, April I0, 1972. K. Nakamoto, Supraventicular parasystole 453 pacemaker and protecting it during all phases of the ectopic cycle, i. e., during both its refractory phase and its apparent non-refractory phase.Electrocardiographic criteria of parasystole given by Chung (1968) include the following, a) varying coupling interval, b) constant shortest interectopic interval and c) frequent appearance of fusion beats. Also yon Knorre and Hafemeister (1969) post...