1. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked between pairs of cultured neurones from either hippocampus or spinal cord were examined using the tight-seal whole-cell recording technique. 2. Using the selective N-methyl-D-aspartate (NMDA)-receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), two components of the EPSP could be resolved in cultures from both brain regions. The APV-sensitive (slow) component had the same latency, but a much slower time-to-peak and longer duration than the APV-resistant (fast) component. Other NMDA antagonists such as ketamine also selectively blocked the slow component of the EPSP. 3. In Mg2+-free medium, the dual-component EPSP had a duration lasting up to 500 ms, greatly exceeding the membrane time constant of the postsynaptic neurone, suggesting that persistent activation of NMDA receptors was responsible for the long duration of the APV-sensitive component. 4. Under voltage clamp the excitatory postsynaptic currents (EPSCs) also showed fast and slow components, both of which had a reversal potential near 0 mV in physiological saline. The synaptic current could be fitted with a sum of two exponentials with a decay time constant for the slow EPSC near 80 ms. The slow current contributed approximately 50% of the total charge transfer during the EPSC. 5. In Mg2+-containing medium, the peak of the fast component was voltage insensitive, whereas the synaptic current measured at a latency of 10-50 ms was voltage dependent with a region of negative slope conductance at membrane potentials hyperpolarized to -30 mV. 6. Raising [Ca2+]o from 1 to 20 mM resulted in a shift of the reversal potential of the APV-sensitive component from near 0 mV to + 10 mV, but the reversal potential of the fast component remained near 0 mV. This suggests that conductances with different ionic permeability underlie the two components of the EPSC and that the slow component is highly permeable to Ca2+ as well as to monovalent cations. 7. Our results demonstrate that two functionally distinct excitatory amino acid receptor channels are simultaneously activated by transmitter release from a single presynaptic neurone. The conductance mechanism underlying the slow component of the EPSP displays the voltage dependence and Ca2+ permeability expected for NMDA-receptor channels. We suggest that the available conductance generating the slow EPSP may be sufficient, even at low firing rates, to influence excitability on both a short-term and more long-lasting basis.