(-)-Cromakalim, a typical K+-channel opener, prevents neuronal death induced by either glucose and oxygen privation or by high (100 microM) extracellular glutamate in primary cultures of hippocampus. (-)-Cromakalim has no effect on the earliest events associated with exposure to glutamate. It does not prevent the rapid rise of intracellular Ca2+, the initial swelling of neurons, or the induction of c-fos mRNA transcription. (-)-Cromakalim inhibits all delayed effects associated with the excitotoxic effect of glutamate: (a) (-)-cromakalim inhibits the late and major phase of intracellular Ca2+ increase occurring up to hours after glutamate application; and (b) although (-)-cromakalim cannot prevent the initial cellular swelling induced by glutamate, cells that have been pretreated with (-)-cromakalim return to their original size in a few hours, whereas non-(-)-cromakalim-treated cells remain swollen for more prolonged periods. Many neurons surviving the initial necrotic phase of glutamate-induced cell death undergo progressive DNA cleavage leading to apoptosis. This apoptotic process is prevented completely by (-)-cromakalim. Glibenclamide, a potent blocker of the ATP-sensitive K+ channel, abolishes all the beneficial effects of (-)-cromakalim. These findings strongly suggest that (-)-cromakalim has postsynaptic effects that are closely related to the regulation of Ca2+ homeostasis and cell volume.