Investigated are rabbit atrial trabeculae (2-4 mm length, 150-800 micrometers diameter). When using a standard microelectrode technique, the action potentials after an intracellular 4-AP and TEA load are prolonged. Only the TEA load significantly diminishes the resting potential of the beating preparations. The typical configuration of the action potential following a period of rest (rapid initial repolarization, secondary depolarization and a late retarded repolarization) is not changed qualitatively. During a repetitive stimulation after a pacing pause the action potentials' restitution is accelerated at 4-AP. After 4-AP and TEA load the secondary depolarization of the first post rest action potential is more accentuated. By reconstructing the post rest action potentials a 4-AP sensitive current is determined. At constant drive this current shows a threshold potential at -40 and a reversal potential at -10 mV. The current is inward directed between -40 and -10 mV and is outward directed at more positive potentials than -10 mV. After a pacing pause the current-voltage relation is shifted to more negative potentials. The 4-AP sensitive current is modelled by consideration of a slow activated (during the action potential) and deactivated (during the pacing pause) conductivity. Processes of accumulation are discussed.