Conventional microelectrode recordings combined with enzymatic cell dispersion methods and a single microelectrode voltage-clamp technique were used to record transmembrane action potentials and ionic currents in isolated single myocytes and in excised segments of human right atrium. Recordings of the outward current(s), which is responsible for the resting potential and early repolarization of the action potential in human right atrium, consistently showed that this tissue has 1) a relatively small inwardly rectifying background potassium current (IK1) which generates the resting potential in mammalian ventricular tissue and Purkinje fibers, and 2) a large time- and voltage-dependent, but Ca2(+)-independent, transient outward current. A somewhat similar K+ current was originally described in neurons and recently has also been identified in a variety of mammalian cardiac tissues. As expected from previous work, this transient outward current in human atrium is blocked by 4-aminopyridine (4-AP; 0.5 mM) and exhibits time- and voltage-dependent inactivation and reactivation. Measurements of action potential shape changes and phasic tension as a function of stimulus frequency, or after 4-AP application, show that in human atrium this current can produce pronounced changes in both the early repolarization of the action potential and force generation.