- Depolarizing afterpotentials (DAPs) were studied in intracellular recordings from neocortical slices bathed in tetrodotoxin (TTX) (1 μM) and tetraethylammonium chloride (TEA) (24 mM), to block voltage-dependent Na + currents and most K + currents. The DAP was Ca 2+ -dependent, in that its magnitude varied as a function of the duration of the preceding Ca 2+ plateau. It had an apparent reversal potential of between −40 and −5 mV. The DAP was blocked when choline replaced all extracellular Na + ; there was a hyperpolarizing shift in apparent reversal potential when extracellular Na + was lowered. The DAP was blocked by amiloride (1 mM), which also decreased the preceding Ca 2+ plateau. The data are consistent with the hypothesis that the DAP is due to electrogenic Na + Ca 2+ exchange.