1. The patch clamp technique, together with intracellular perfusion of the catalytic fragment of protein kinase C (PKCM), was employed to investigate the role of this enzyme in the intracellular regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptors in cultured hippocampal neurones. 2. The responses evoked by near-maximal concentrations of kainate (250 microM) and AMPA (100 microM) were potentiated by the introduction of PKCM, whilst co-application of the inhibitory peptide fragment PKCI(19-36) prevented this action. 3. Modulation of kainate responses by PKCM was dependent upon the concentration of agonist applied. Currents evoked by kainate were potentiated at concentrations above those which caused 50% of the maximal response (EC50) and depressed at lower concentrations. Furthermore, okadaic acid, a specific inhibitor of phosphatases 1 and 2A, had a similar effect upon concentration-response relationships when currents activated by kainate were recorded using the perforated patch technique. 4. In addition, the mean amplitude and/or time constant of decay of miniature excitatory synaptic currents (mediated by AMPA/kainate receptors) was increased by the intracellular injection of PKCM. 5. These observations suggest that the function of postsynaptic excitatory amino acid receptors can be modulated by the activity of PKC as well as by endogenous phosphatases. This regulation may contribute to some forms of synaptic plasticity within the central nervous system.