Neurons acutely isolated from the CA1 region of rat hippocampal slices using the 'vibrodissociation' method were voltage-clamped in the whole-cell configuration. The currents through NMDA channels were elicited by application of 100 microM aspartate (ASP) in a Mg2+-free solution in the presence of 3 microM glycine. The compound KB-R7943, (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate) known as a Na+/Ca2+ exchange inhibitor was able to block effectively the NMDA channels. At a holding potential of -100 mV, the measurement of the concentration dependence of the stationary current blockade revealed the existence of two populations of NMDA channels differing by a high (IC50 = 0.8 microM) and low (IC50 = 11 microM) affinity for KB-R7943. The Hill coefficients indicated that one blocking molecule can bind to NMDA channels which have a high affinity for KB-R7943 and at least two blocking molecules can bind to the NMDA channels which have a low affinity for KB-R7943. When applied externally, KB-R7943 can bind to the low-affinity NMDA channels irrespective of whether or not these channels are activated by the agonist. The KB-R7943-induced blockade of the NMDA channel was partly voltage-dependent. Within the framework of the Woodhull model, the apparent value of delta calculated for the voltage-dependent binding of KB-R7943 was in the range of 0.26-0.41. The blocking action of KB-R7943 on NMDA channels did not depend either on ASP or glycine concentrations which indicated that the binding sites for KB-R7943 and those for the agonist and the coagonist did not overlap.