The effects of the hypoglycemic sulphonylurea tolbutamide, a marker of K(+)-ATP channels, on the N-methyl-D-aspartate- (NMDA) and kainate-activated currents were studied in rat hippocampal neurons in culture, using the patch-clamp technique in a whole-cell configuration. Tolbutamide (500 microM) reversibly increased the peak amplitude and the steady state level of NMDA- but not kainate-evoked currents. This effect was not glycine dependent as it was observed at low and saturated concentrations of glycine. The affinity of the NMDA receptor-channel complex for glycine did not change in the presence of tolbutamide. The action of tolbutamide on the NMDA-activated current was not mediated by K(+)-ATP channels since CsCl was added intracellularly at concentrations which completely blocked all K+ channels. Possible mechanisms explaining the effect of tolbutamide via the modulation of intracellular messengers are discussed.