Rat cortical neurons grown in cell culture were exposed to 500 μM glutamate for 5 min during continuous current recording from cell-attached patches. The Ca2+-dependence and ion selectivity of the membrane channels activated during and after glutamate application were studied in inside-out patches. Glutamate blocked spontaneous action potential firing. In 77% of the experiments glutamate activated several types of ion channels indirectly, i.e. via a change of cytoplasmic factors. Channel activity did not disappear after removing glutamate from the bath. A K+ channel requiring intracellular calcium ([Ca2+]i) was activated in 44% of the experiments (conductance for inward currents in cell-attached patches 118 ± 6 pS;‘BK channel'). Another Ca2+-dependent channel permeable for Cl- (conductance for outward currents in cell-attached patches 72±17 pS), acetate and methanesulphonate appeared in 26% of the patches. Other K+ channels of smaller conductance were infrequently observed. During and after glutamate application the activity of the BK channel showed an initial increase followed by a transient decay and a second rise to a plateau, probably reflecting a similar time course of changes in [Ca2+]i. Both phases of increasing channel activity required the presence of extracellular Ca2+ suggesting that [Ca2+]i was mainly increased by Ca2+ influx. The N-methyl-d-aspartate (NMDA) antagonists dizocilpine (MK-801, 10 μM) and dl-2-amino-5-phosphonovaleric acid (AP5; 100 μM), added within 5 min after glutamate application, stopped BK channel activity and restored the spontaneous action potential firing. We conclude that the influx of Ca2+ through NMDA receptor channels causes a strong activation of Ca2+-dependent K+ channels, which is likely to result in pronounced loss of intracellular K+. NMDA receptor channels seem to remain active for a long time (>10 min) after the end of glutamate application.