The inhibitory glycine receptor (GlyR) is composed of polypeptide subunits that contain intracellular consensus sequences for phosphorylation by protein kinase C (PKC). During whole-cell recording from rat hippocampal neurones, we observed a time-dependent increase of the glycine-induced membrane current. After 22 min the amplitude was 260 + 13% of the initial control response. PKC was involved in the modulation of hippocampal glycine receptors, since the observed effect was more prominent when the phorbol ester PMA, an activator of PKC, was included in the patch pipette. The action of PMA was mimicked by applying the '5-HT2 receptor agonist, alpha-methyl-serotonin, to the cells. The time-dependent increase in glycine responses was reduced by either tamoxifen, an inhibitor of PKC, or by alkaline phosphatase. Protein kinase A and tyrosine kinase were not involved as modulatory drugs of these kinases had no effect. These results provide direct evidence for the regulation of GlyR function by PKC in rat hippocampal neurones.