The release of ATP was studied in cultures of astrocytes derived from the brain hemispheres of newborn rats. There was a basal efflux of ATP, which was increased up to 19-fold by glutamate (300–1000 μM), N-methyl- d-aspartate (20–500 μM), α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 30–100 μM) and kainate (20 μM). The N-methyl- d-aspartate receptor-selective antagonist 2-amino-5-phosphonopentanoate (100 μM) blocked the effect of N-methyl- d-aspartate but not the effects of AMPA, kainate and glutamate. The AMPA receptor-selective antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (30 μM) blocked the effect of AMPA and also of glutamate and N-methyl- d-aspartate, but not the effect of kainate. The kainate receptor-selective antagonist d-glutamyl-amino-methanesulfonate (30 μM) blocked the effect of kainate but not of glutamate. Glutamate (1000 μM) did not increase the release of lactate dehydrogenase from astrocytes. Excitatory amino acids are known to release adenyl compounds in the brain. The present results identify one adenyl compound thus released, namely ATP, and identify astrocytes as one source. The release is brought about by activation of any of the three ionotropic glutamate receptor types— N-methyl- d-aspartate, AMPA and kainate receptors. AMPA receptors seem to mediate at least a part of the effect of glutamate itself, but the involvement of other receptors cannot be ruled out. ATP and its degradation products, such as adenosine, once released, may exert acute as well as trophic effects on neurons and glial cells.