1. The underlying ionic mechanisms of the postspike depolarizing afterpotential (DAP) in hippocampal dentate granule (DG) neurons of immature rats (postnatal 7- to 17-day-old) were examined using whole cell patch recordings in brain slices. 2. In current-clamp mode, the DAP followed each single action potential. Graded DAP-like responses were also evoked by depolarizing current pulses when the action potential was blocked by tetrodotoxin (TTX), demonstrating that the TTX-sensitive Na+ conductance is not necessary for DAP generation. The membrane resistance near the DAP peak was lower than at rest, suggesting activation of inward currents rather than blockade of outward currents during the DAP. The DAP peak amplitude showed a strong dependence on voltage, increasing with membrane hyperpolarization and decreasing with depolarization in the range of -90 to -50 mV. Repetitive stimulation at 1-2 Hz did not change the amplitude or decay of the DAP or DAP-like response. 3. Bath application of 2 mM 4-aminopyridine (4-AP) and 5 mM tetraethylammonium chloride (TEA) prolonged the action potential and enhanced the DAP, suggesting that the DAP waveform is determined by the interaction of voltage-activated outward K+ currents and inward currents. 4. Bath application of 2 mM Co2+ depressed the DAP and the DAP-like potential. Replacement of extracellular Ca2+ with Ba2+ potentiated the DAP. Intracellular Ca2+ chelation with the fast chelator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA), only slightly enhanced the DAP, suggesting that the DAP is not generated by inward currents activated secondary to Ca2+ influx.(ABSTRACT TRUNCATED AT 250 WORDS)