To explore the mechanisms underlying the shortening of the cardiac action potential in hypoxia, we studied the effect of hypoxia on the ionic currents in cat papillary and trabecular muscles using the single sucrose gap-voltage clamp technique. For potentials positive to −70 mV, hypoxia induces an increase in time-independent outward current. The changes in the tail current suggest that time-dependent outward current is not increased but, rather, reduced. Because the time course of IK remains unchanged, we concluded that the shortening of the action potential is not a result of a change in the time-dependent outward current. In the potential range of the plateau, the amplitude of the slow inward current is not affected by hypoxia. Its time constant of inactivation appears slightly decreased. The prolongation of the action potential by epinephrine during hypoxia is accompanied by an increase in the slow inward current. As a result of these studies, we conclude that the shortening of the cardiac action potential in the early stage of hypoxia results from an increase in K+outward background current.Circ Res 47: 501-508, 1980