Abstract: Subchronic treatment with haloperidol increases the number of asymmetric glutamate synapses associated with a perforated postsynaptic density in the striatum. To characterize these synaptic changes further, the effects of subchronic (28 days) administration of an atypical antipsychotic, clozapine (30 mg/kg, s.c.), or a typical antipsychotic, haloperidol (0.5 mg/kg, s.c.), on the binding of [3H]MK-801 to the NMDA receptor-linked ion channel complex and on the in situ hybridization of riboprobes for NMDAR2A and 2B subunits and splice variants of the NMDAR1 subunit were examined in striatal preparations from rats. The density of striatal glutamate immunogold labeling associated with nerve terminals of all asymmetric synapses and the immunoreactivity of those asymmetric synapses associated with a perforated postsynaptic density were also examined by electron microscopy. Subchronic neuroleptic administration had no effect on [3H]MK-801 binding to striatal membrane preparations. Both drugs increased glutamate immunogold labeling in nerve terminals of all asymmetric synapses, but only haloperidol increased the density of glutamate immunoreactivity within nerve terminals of asymmetric synapses containing a perforated postsynaptic density. Whereas subchronic administration of clozapine, but not haloperidol, resulted in a significant increase in the hybridization of a riboprobe that labels all splice variants of the NMDAR1 subunit, both drugs significantly decreased the abundance of NMDAR1 subunit mRNA containing a 63-base insert. Neither drug altered mRNA for the 2A subunit, but clozapine significantly increased hybridization of a probe for the 2B subunit. The data suggest that some neuroleptic effects may be mediated by glutamatergic systems and that typical and atypical antipsychotics can have varying effects on the density of glutamate in presynaptic terminals and on the expression of specific NMDA receptor splice variant mRNAs. Alternatively, NMDAR1 subunit splice variants may differentially respond to interactions with glutamate.