1. Using whole-cell patch-clamp recordings of NMDA EPSCs from co-cultured rat hippocampal (CA region) neurones, developmental changes in the kinetic and pharmacological properties of synaptic NMDA receptors were investigated. During in vitro differentiation a fast decaying component increasingly contributed to NMDA EPSCs. 2. Extracellular Mg2+ (1 mM) strongly blocked NMDA EPSCs at all stages in culture. Using the NR2B subunit-specific NMDA receptor antagonist ifenprodil (3 microM), we observed a developmental decrease in ifenprodil sensitivity of NMDA EPSCs. This suggests developmental changes in the expression of NMDA receptor subtypes. 3. To transiently block presynaptic exocytosis, we incubated presynaptic explants with tetanus toxin (TeTx) prior to cultivation. In TeTx-pretreated cultures the occurrence of fast decaying components of NMDA EPSCs and the developmental decrease in ifenprodil sensitivity was inhibited. Our results indicate a regulatory role of presynaptic exocytosis in the expression of NMDA receptor subtypes.