Single cells were isolated from human atria. The ionic currents in these cells were measured using the whole cell clamp technique. Time-dependent currents during depolarizing clamp steps can be described as due to Ca current and transient outward current. Time-dependent inward currents were seen during hyperpolarizing voltage steps. These currents are carried by the inward rectifier iK1, while in some cells also an if pacemaker current is present. The effect of acetylcholine was investigated at potentials negative to -50 mV. Acetylcholine induces an inwardly rectifying K current. The acetylcholine-induced current is different from iK1 in kinetics and Ba-sensitivity. The effect of acetylcholine decreases in time due to desensitization. The electrical response of the human atrial cells to acetylcholine is qualitatively similar to the effect of acetylcholine in guinea-pig atrial cells.