To determine whether membranes of mammalian central neurons contain an ATP-sensitive K+ (KATP) channel similar to that present in pancreatic beta cells, the patch-clamp technique was applied to cultured neurons prepared from the neonatal rat cerebral cortex and hippocampus. In whole-cell experiments with hippocampal neurons, extracellular application of 0.5 mM diazoxide (a KATP channel activator) elicited a hyperpolarization concomitant with an increase in membrane conductance, whereas application of 0.5 mM tolbutamide (a KATP channel blocker) induced a depolarization with a decrease in conductance. Similar results were obtained with cortical neurons. In outside-out patch experiments with cortical neurons, a K+ channel sensitive to these drugs was found. The channel was completely blocked by 0.5 mM tolbutamide and activated by 0.5 mM diazoxide. The single-channel conductance was 65 pS under symmetrical 145 mM K+ conditions and 24 pS in a physiological K+ gradient. In inside-out patch experiments, this channel was demonstrated to be inhibited by an application of 0.2-1 mM ATP to the cytoplasmic surface of the patch membrane. These results indicate that the membranes of rat cortical neurons contain a KATP channel that is quite similar to that found in pancreatic beta cells. It is also suggested that the same or a similar K+ channel may exist in membranes of hippocampal neurons.