Factors that facilitate the occurrence of cholinergic atrial arrhythmias were studied on 16 rabbit biatrial preparations in spontaneous rhythm. Sinus cycle length and characteristics of the atrial action potential were measured by the microelectrode technique in the basal state, then in the presence of acetylcholine at a concentration of 1.4 x 10(-5) M. Induction of arrhythmia was attempted by programmed stimulation, using an increasing number of extrastimuli. In the presence of acetylcholine the sinus cycle length increased by 106 +/- 63 p. 100 (p less than 0.0001) and the action potential duration, measured at 90 p. 100 repolarization (APD90) decreased from 60 +/- 15 ms to 40 +/- 11 ms (p less than 0.001). Reentrant activities, which had not been found in the basal state, were induced in 5 preparations. Under acetylcholine the sinus cycle of inducible preparations was shorter than that of non inducible preparations (663 +/- 272 ms vs 1218 +/- 531 ms, p less than 0.05). The percentage of sinus cycle lengthening was significantly smaller in inducible preparations (54 +/- 31 p. 100 vs 129 +/- 60 p. 100, p less than 0.05). Although sinus cycle lengthening was different in the two types of preparations, the APD90 was shortened in the same proportions. The vulnerability of the preparations seemed to depend mainly on a frequency effect. Vagal atrial arrhythmias occurred with a relatively small reduction in sinus rhythm. It is probable that an overpotent vagal effect is less arrhythmogenic because of its more homogeneous action on tissues.