1. Excitatory postsynaptic potentials (EPSPs) and their underlying currents (EPSCs) were recorded from dentate granule cells in thin hippocampal slices of rats using the tight-seal whole-cell recording technique. 2. At resting membrane potentials (ca -60 to -70 mV), the EPSCs clearly consisted of a dominant fast and a smaller slow component. The slow EPSC component markedly increased with depolarization. This resulted in a region of negative slope conductance (between -50 and -30 mV) in the peak current-voltage (I-V) relation of the dual-component EPSC in most neurones. The EPSCs reversed entirely at -1.2 +/- 2.8 mV (n = 15). 3. Using selective antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptors, two pharmacologically distinct components of the natural EPSCs were isolated. The non-NMDA EPSCs displayed a linear I-V relation. Their rise times (0.5-1.9 ms) were independent of membrane voltage but seemed to depend critically on the precise dendritic location of the synapse. Their decay was approximated by a single exponential with a time constant ranging from 3 to 9 ms. The time course of these EPSCs was independent of changes in extracellular Mg2+. 4. The NMDA EPSCs displayed a non-linear I-V relation. At resting membrane potentials their peak amplitudes were 20 pA and increased steadily with depolarization to -30 mV. At membrane voltages positive to -30 mV the peak I-V relation was linear. The rise times of NMDA EPSCs ranged from 4 to 9 ms and were insensitive to membrane voltage. 5. The NMDA EPSCs decayed biexponentially. Both time constants, tau f and tau s, increased with depolarization in an exponential manner, tau s being more voltage dependent than tau f. Lowering extracellular Mg2+ slightly reduced both rate constants but did not completely abolish their voltage sensitivity. 6. Bath application of NMDA to outside-out patches from granule cells induced single channel currents of 52 pS in nominally Mg(2+)-free solutions. They displayed a burst-like single-channel activity with clusters of bursts lasting several hundreds of milliseconds. Currents through single NMDA receptor channels reversed around 0 mV. 7. The fractional contributions of NMDA and non-NMDA components to peak currents and synaptic charge transfer were assessed. At resting membrane potential the NMDA EPSC component accounted for 23% of the peak current and for 64% of the synaptic charge transfer. The contribution of the NMDA EPSC component to the synaptic charge transfer strongly increased with small depolarizations from rest.