Evoked excitatory postsynaptic currents (EPSCs) were recorded from the pyramidal neurons of layer 2/3 of the somatosensory cortex in the slices obtained from the brain of humanely killed 17- to 22-day-old mice. EPSCs were elicited by electrical stimulation of vertical axons originating from layer 4-6 at 0.1 Hz, in the presence of picrotoxin (100 μM) and 6-cyano-7-nitroquinoxaline-2,3-dione (30 μM). The current measured at −80 mV was mediated mostly by P2X receptors, whereas current through NMDA receptors was the dominant component of synaptic currents measured at −40 mV or 40 mV [1]. Statistical significance of data obtained was verified using Student’s paired t test with P < 0.05. Low frequency stimulation at −80 mV caused a strong inhibition of NMDA component of EPSC that was subsequently measured when switching the holding potential to −40 or 40 mV. This suppression of the NMDA component of the EPSC was abolished when intracellular solution was supplemented with BAPTA (3 mM). The inhibition of the NMDA receptor-mediated component of the EPSC was also decreased by the P2X receptor antagonist PPADS (63 ± 22%, n = 10), suggesting that NMDA receptors were inhibited following activation of P2X receptors. Direct application of P2X receptor agonists to the cortical slice also caused inhibition of current through NMDA receptors. When ATP (10 μM) or αβ-methylene-ATP (10 μM) was applied for 60 s when holding the cell at −80 mV, subsequently elicited NMDA receptor-mediated EPSCs measured at −40 mV were inhibited. ATP inhibited the NMDA receptor-mediated EPSCs by 43 ± 24% (n = 12), whereas αβ-methylene-ATP inhibited 26 ± 10% (n = 9). When these P2X receptor agonists were applied at 40 mV, they did not inhibt the subsequently elicited NMDA receptor-mediated EPSC. The inhibition of the NMDA current following activation of P2X receptors was also observed in acutely isolated pyramidal neurons. ATP (10 μM, 5 s) or αβ-methylene-ATP (10 μM, 5 s) reduced the current evoked by subsequent application of NMDA (100 μM) by 25 ± 13% (n = 5). The suppression of evoked NMDA currents did not occur when Ca²⁺ in the extracellular solution was substituted by Ba²⁺. Taken together these results suggest that postsynaptic P2X receptors can control the activity of NMDA receptors via a calcium-dependent mechanism. This may imply a modulatory role for the purinergic component of excitatory synaptic transmission.