We used standard microelectrode techniques to study Tyrode's-superfused human atrial fibers obtained at cardiac surgery. Two types of sustained rhythmic activity occurred. One resulted from slow phase 4 depolarization and had a spontaneous rate = 20-26 beats/min. Epinephrine increased and the slow channel blockers, AHR-2666 (AHR) and verapamil decreased both phase 4 slope and spontaneous rate. Acetylcholine (ACh) and lidocaine decreased the slope of phase 4, but the slowing of rate was less than that induced by AHR and verapamil. Tetrodotoxin (TTX) also decreased the slope of phase 4 and spontaneous rate, to an extent that was intermediate between the actions of AHR-verapamil and ACh-lidocaine. A second type of sustained rhythmic activity was triggered by delayed afterdepolarizations (DAD). DAD amplitude increased as stimulus cycle length decreased and, at critical cycle lengths, DAD initiated trains of spontaneous action potentials at rates > 70 beats/min. Spontaneously occurring DAD were suppressed by AHR and were transiently diminished by ACh. This effect of ACh was accompanied by hyperpolarization of the fibers. DAD also were induced by epineph-rine. These DAD were unaffected by TTX, lidocaine, or ACh and were suppressed by AHR and verapamil. In summary, the slow inward current contributes to the sustained rhythmic activity that occurs with automaticity or DAD in human atrium. A TTX-sensitive current also contributes to automaticity. DAD that occur spontaneously are largely insensitive to the effects of agents that increase K conductance (although ACh has a transient effect) and those that are induced in the presence of epinephrine do not respond to agents which increase K conductance (ACh, lidocaine) or TTX.Circ Res 47: 267-277, 1980