1. The effect of docosahexaenoic acid (DHA) on N-methyl-D-aspartic acid (NMDA) responses in the presence of glycine was investigated in pyramidal neurons acutely dissociated from rat cerebral cortex in whole-cell and single channel configurations. 2. DHA potentiated the NMDA-induced response but reduced the non-NMDA (kainate-induced) response in a concentration-dependent manner at a holding potential of -60 mV under voltage-clamp conditions. 3. Arachidonic acid (AA) also potentiated the NMDA-induced response in a manner similar to DHA. Oleic acid caused a slight potentiation. However, other polyunsaturated and saturated fatty acids had no such effects. 4. The facilitatory action of DHA on the NMDA-induced response was not affected by adding inhibitors of cyclo-oxygenase, lipoxygenase or phospholipase A2, suggesting that DHA may exert its facilitatory effect directly on the NMDA receptor. 5. The facilitatory action of DHA was observed in the presence of a saturating dose of NMDA. Moreover, a detailed analysis of the NMDA receptor-operated single channel currents revealed that, in the presence of DHA, the open probability of the channel increased without changing the conductance, indicating that DHA may act by binding directly to a novel site on the NMDA receptor or by altering the lipid environment of the NMDA receptor and thereby potentiating the response to NMDA. 6. The results are discussed in terms of the possibility that DHA may play an important role in the genesis of long-term potentiation, at least that involving the activation of NMDA receptors.