Neurotransmission between neurones and glial cells is an important element of integration in the central nervous system [1]. Glial cells express multitude of neurotransmitter receptors able to sense neuronal activity [2, 3]. Here we report a study of NMDA-activated currents in cortical astrocytes. The experiments were performed on neurones from transgenic mice, in which astrocytes expressed enhanced green fluorescent protein (eGFP) [4]. Astrocytes were studied by whole-cell voltage-clamp either in a slice preparation or after acute isolation using vibro-dissociation [5]. Astrocytes were identified by their fluorescence, and by the expression of passive potassium currents often regarded as a specific electrophysiological signature. Concentration clamp experiments performed on acutely isolated astrocytes (obtained from humanely killed mice) demonstrated two pharmacologically distinct components of glutamate-induced currents. 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), an inhibitor of AMPA and kainate receptors, suppressed the initial rapid part of the glutamate-induced current; the IC50 was 2.2 ± 0.4 μM (mean ± S.D., n = 4). The slower component of the glutamate-evoked current was inhibited by the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP-5); the IC50 was 0.6 ± 0.1 μM (n = 5). NMDA evoked inward currents in isolated astrocytes; the EC50 was 0.34 ± 0.06 μM (n = 6). This current was blocked by 5H-dibenzo[a,d]cyclohepten-5,10-imine, 10,11-dihydro-5-methyl-, (5S)-, (2Z)-2-butenedioate (1:1)(MK801) in an apparently use-dependent manner. The NMDA-evoked currents in astrocytes demonstrated linear I-V relation, which were unaffected by the extracellular magnesium concentration (0 to 10 mM). Electrical stimulation of neurones located in layer V of the cortex induced a complex inward current in astrocytes situated in the cortical layer III. Part of this current response was sensitive to block by MK801, suggesting the expression of synaptically activated NMDA receptors in astroglia. We conclude that astrocytes express functional NMDA receptors which do not show magnesium block, and which are involved in neuronal-glial neurotransmission.