The pyramidal neurons in the CA1 area of hippocampal slices from 17- to 19-day-old rats have been investigated by means of patch clamp. Excitatory postsynaptic currents (EPSCs) were elicited by stimulating the Schaffer collateral at a frequency below 0.2 Hz. It was found that inhibition of glutamatergic transmission by 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 100 microM 2-amino-5-phosphonovaleric acid (D-APV) left a small component of the EPSC uninhibited. The amplitude of this residual EPSC (rEPSC) comprised 25 +/- 11% of the total EPSC when measured at a holding potential of -50 mV. The rEPSC was blocked by selective P2 blocker pyridoxal phosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS) 10 microM and bath incubation with non-hydrolysable ATP analogues, ATP-gamma-S and alpha, beta-methylene-ATP at 50 and 20 microM, respectively. The rEPSC was dramatically potentiated by external Zn2+ (10 microM). In another series of experiments exogenous ATP was applied to the CA1 neurons in situ. An inward current evoked by ATP was inhibited by PPADS to the same extent as the rEPSC. It is concluded that, depending on membrane voltage, about one-fifth to one-quarter of the EPSC generated by the excitatory synaptic input to the hippocampal CA1 neurons of rat is due to the activity of P2X receptors.