Atrial trabeculae (studied in voltage-clamping conditions and in the presence of 0.5 mmol/l BaCl2 to abolish gK1) responded to 1 s hyperpolarizations to beyond approximately E = -140 mV (from HP of about E = -80 mV) with an inwardly directed current increasing with time. Quite similar results were obtained with enzymatically dissociated frog atrial cells studied in whole cell voltage clamp with a patch-clamp pipette. This behaviour could be accounted for by assuming the presence of an "if" current at this quite negative range of potentials or by the fact that the cell membrane may undergo reversible electropermeabilization when its potential is brought to values negative to about -140 mV (Stämpfli 1958). When a brief (1 ms) and large (150 mV) hyperpolarization was applied 1 s before the test pulse, an inwardly directed current increasing with time was elicited by test pulses to beyond approximately E = -120 mV. This current was neither abolished in the presence of 1 mmol/l CsCl nor greatly reduced in the absence of Na+ ions, unlike "if" (Di Francesco 1981). We conclude that this current having a time course similar to that of "if" is of different nature and we argue that it might be accounted for by electropermeabilization of the membrane (reversible within about 2.5 min) due to the electrical shock represented by a brief and large hyperpolarization.