The authors examined tachycardias induced by administering acetylcholine (AC), 1-2 micrograms into the artery of sinus node (ASN) of an isolated specimen of the canine right atrium, which had spontaneous automatism. Bilateral multielectrode mapping was employed. The episodes of tachycardia occurred during AC-induced arrest of sinus rhythm. In 81% of the cases, episodes of arrhythmia consisted of 2-3 beats, in 19%, 6-150 beats. The mapping revealed a focal picture of activation during short-term episodes of arrhythmia and transition from a focal type of activation to the re-entry in most cases of "prolonged" episodes. With this, excitation circulation might be detected only on one side of the specimen, in the presence of the focal activation picture, on the other. Focus-re-entry transition is proposed to be caused by a mechanism associated with heterogeneous refractoriness of atrial tissue.