IN the cerebellum, NMDA (N-methyl-D-aspartate) receptors play an important role in neuronal differentiation(1,2) and excitatory synaptic transmission(3-5). During early cerebellar development, marked changes occur in the distribution of messenger RNAs encoding various NMDA-receptor subunits(6). To determine whether these changes result in the appearance of functionally distinct NMDA receptors(7-9), we have recorded single-channel currents in rat cerebellar granule cells during the period of their migration from the external germinal layer to the inner granular layer(10). Here we show that before synapse formation(10,11), pre-migratory and migrating granule cells express NMDA receptors possessing single-channel properties similar to those previously described for many central neurons(12,16). In contrast, mature post-migratory cells also express an atypical form of NMDA receptor that has a lower single-channel conductance and distinct kinetic behaviour. The properties of these ’low-conductance’ channels correspond to those described(17) for recombinant NMDA receptors formed by coexpression of NR1 and NR2C subunits(8,9). The NR2C subunit appears postnatally and is found predominantly in the adult cerebellum(6-8). Our data demonstrate developmental changes in NMDA-receptor properties at the single-channel level, and suggest that is the cerebellum the expression of a specific subunit protein results in a distinct form of native receptor.