1. Pharmacologically isolated miniature NMDA receptor-mediated excitatory postsynaptic currents (mN-EPSCs) were recorded in large visual cortical neurons in layer V of rat cortical slices. Haloperidol (100 microM) and CP101,606 (10 microM), two specific blockers of NMDA receptors comprising NR1/NR2B subunits, were tested on mN-EPSCs in rats at postnatal days 7 and 8 (P7-P8) and P13-P15. At both ages tested, no significant effects of these drugs were seen in the whole population of neurons, although in few neurons at both ages changes in amplitude were observed with haloperidol. Other dopamine receptor antagonists, spiperone and clozapine, failed to decrease mN-EPSCs in cortical neurons at P13-P15. 2. CP101,606 (10 microM) significantly decreased the amplitude of evoked N-EPSCs (eN-EPSCs) in visual cortical slices from rats at P3-P5, a developmental stage at which mRNA studies have indicated the virtual absence of NR2A mRNA. CP101,606 failed to significantly change evoked AMPA-mediated EPSCs at P5 and eN-EPSCs at P7-P8 and P13-P15. 3. NMDA receptor-mediated currents were also studied in somatic outside-out patches at P13-P15 with fast application of L-glutamate (1 mM). Haloperidol (50 microM) and CP101,606 (10 muM) blocked these currents in all patches tested. The effect of CP101,606 was concentration dependent. 4. We suggest that rather early in development synaptic receptors comprising NR1/NR2B subunits could be associated with other subunits so that blockade by haloperidol and CP101,606 is prevented. Moreover, the consistent blockade seen in outside out patches might be ascribed to the confinement of NR1/NR2B receptors to an extrasynaptic population.