NMDA-type glutamate receptors (NMDARs) are heteromultimeric channels assembled from NR1 subunits and at least one type of NR2 subunit (2A-D). Purkinje cells (PCs) from young rats contain mRNA for NR1 and NR2D subunits (1), and in these cells presumptive NR1/NR2D receptors in extrasynaptic membrane have been identified on the basis of their gating behaviour and low single-channel conductance (40 and 20 pS; 2). With development, functional NMDARs are lost from rat PCs. By contrast, PCs from adult mice display immunolabelling for NR1, NR2A, -2B and -2C/D subunits (3). Here we have used NR2D knock-out mice (NR2D -/-; 4) to investigate the identity of the NMDARs in mouse PCs. Further, we have examined the conditions under which PC NMDARs may be activated. We recorded NMDA-activated channels in outside-out membrane patches from PCs in slices from wild-type (WT) and NR2D -/- mice of various ages. In P6-9 WT mice, NMDA (10 μM in nominally Mg²⁺-free solution) activated single-channel openings of ~40 and ~20 pS (slope conductance: 41.2±1.7 and 21.0±0.5 pS; mean±S.E.M., n = 3). These low-conductance events were absent from patches taken from NR2D -/- mice (n = 10). These observations are consistent with the view that NMDARs in PC from young rodents are formed from NR1 and NR2D subunits. However, we also observed high-conductance NMDA-activated channels from NR2D -/- mice, suggesting the presence of NMDARs formed from other subunits. In one patch from a young mouse, NMDA-activated channels were found with a slope conductance of ~50 and ~40 pS, suggesting the presence of NR2A- or NR2B-containing NMDARs. Consistent with the formation of functional NR2D-lacking NMDARs in mice, we also observed similar high-conductance channels in a PC from an adult WT mouse. In this case, extracellular Zn²⁺ (5 and 200 nM; 10 mM tricine-buffered solution) blocked the openings with an efficacy that suggested the presence of NR2A-containing NMDARs (5). Although PCs in young mice express NMDARs, it is not clear if they are activated by endogenous glutamate. In this regard, we observed an AP5-sensitive whole-cell current upon application of the glutamate transport blocker (DL-threo-β-benzyloxyaspartate, TBOA 100 μM; 46±14 pA, n = 5; P6-9), suggesting that NR2D-containing NMDARs may be activated by ambient glutamate under certain conditions. Our experiments confirm that the NR2D subunit is required for expression of most NMDARs in Purkinje cells of newborn animals. Our results also show that NMDARs lacking NR2D are also expressed, albeit rarely, in both young and adult mice. We are currently investigating a possible role for NMDAR activation in developing Purkinje cells.