We studied epinephrine-induced delayed afterdepolarizations and triggered activity in atrial fibers from the canine coronary sinus to determine whether their responses to cardiac pacing would aid in formulating a uniform set of guidelines for differentiating this triggered activity from other arrhythmogenic mechanisms. We used standard microelectrode techniques and compared the delayed afterdepolarizations and triggered activity with those occurring in ouabain-superfused Purkinje fibers. Like Purkinje fibers, the frequency of triggering in the coronary sinus and the coupling interval of the first triggered beat were related directly to the basic drive cycle length, and the delayed afterdepolarization amplitude and frequency of triggering were related to the coupling interval of premature stimuli (S2). However, unlike Purkinje fibers, the coupling interval of the delayed afterdepolarization and of the first triggered beat were independent of the S2. Once initiated, triggered activity in the coronary sinus followed one of four rhythm patterns: in all four, the minimum and equilibrium cycle lengths were independent of the initiating cycle length. Triggered activity was terminated by overdrive and S2 pacing, especially by long episodes of overdrive at short cycle length. The first escape beat after overdrive was linearly related to the overdrive cycle length, resulting in overdrive acceleration. The return cycle length after S2 was linearly related to the S2 coupling interval. Because delayed afterdepolarizations and triggered activity in the coronary sinus respond differently to pacing from those in ouabain-superfused Purkinje fibers, triggered activity in general may not be identified by a uniform set of guidelines.