Whole-cell recording techniques were used on single right atrial myocytes to study the ionic currents that may be responsible for the diverse diastolic voltage characteristics of atrial tissue. Ionic currents were activated by hyperpolarizing voltage pulses negative to -30 mV. In general, four different types of cells were identified based primarily on the ionic currents elicited during hyperpolarization. The first cell type exhibited an inward current that decayed with time at more negative voltages, reversed near the potassium equilibrium potential, inwardly rectified at more positive voltages, increased in elevated extracellular potassium, and was blocked by 3 mM barium or 10 mM cesium. This current was identified as the potassium current iK1. A second cell type exhibited a time-dependent inward current that increased at more negative voltages, had an activation range between -50 and -110 mV, had a reversal potential of -26 mV, and was blocked by 3 mM cesium. This current was identified as an if current. A third cell type exhibited an inward current that initially decayed and then became more inward with time. Barium (3 mM) abolished the initial inward current and revealed a time-dependent increasing inward current that was blocked by 3 mM cesium. This current was composed of both the iK1 and if currents. A fourth cell type exhibited only small time-independent leak currents in response to hyperpolarization. These results indicate that individual cells within the right atrium are electrophysiologically heterogeneous with respect to the types of ionic channels present in their sarcolemmal membranes. This specialization in ionic currents partially explains the diverse diastolic voltage characteristics and functional properties of atrial tissue.