We studied the effects of halothane on membrane potentials and ionic currents in single guinea pig atrial and ventricular cells prepared by an enzymatic dispersion procedure. In both atrial and ventricular cells, action potential overshoot and its plateau phase were significantly decreased by halothane (2%) without change in resting potential. However, the duration of the ventricular action potential measured at 90% repolarization was markedly shortened by halothane (2%) (to 60% of control), whereas that of the atrial action potential did not change significantly. Corresponding voltage clamp experiments demonstrated that in atrial cells halothane (2%) significantly depresses the time- and voltage-dependent outward K+ current (IK) (to 46% of control); and that in ventricular cells IK is then nearly absent. In both atrial and ventricular cells halothane had no effect on the inwardly rectifying K+ current (IK1). On the other hand, halothane (2%) decreased the slow inward Ca2+ current (ICa) in both atrial and ventricular cells (to 36% and 29% of control, respectively). The results suggest that the shortened action potential in ventricular cells by halothane may well be responsible for the decrease of the plateau phase resulting from the depression of ICa; and that in atrial cells the depression of IK and ICa by halothane had no significant effect on the duration of action potential.