At synapses between parallel fibres and molecular layer interneurons (INs; basket and stellate cells) in the cerebellum, the inwardly rectifying current-voltage (I-V) relationships of excitatory postsynaptic currents (EPSCs) become linear following high-frequency stimulation (Liu & Cull-Candy, 2000). It has been suggested that this reflects an activity-dependent targeting of GluR2 containing AMPARs to the synapse, and a reduction in Ca²⁺-permeability of the postsynaptic receptors. We have now examined whether a change in the contribution of GluR2-containing AMPARs to EPSCs may also occur at these IN synapses during development. Rats were anaesthetized with isoflurane and humanely killed. Whole-cell recordings were made from INs in coronal cerebellar slices (in the presence of NMDAR- and GABA_AR- antagonists). Spermine was included in the patch-pipette solution to identify the presence of GluR2-lacking synaptic AMPARs (Kamboj et al. 1995). At postnatal days (P) 7-8, EPSCs displayed a strong inwardly rectifying I-V relationship, signified by a low Rectification Index (RI) of 0.30±0.04 (n=23; mean±S.E.M.). This is indicative of the presence of Ca²⁺-permeable AMPARs. By P17 (n=19) and P28 (n=8), the relationship was considerably less rectifying, with RI values of 0.55±0.05 and 0.49±0.06, respectively. Rectification values at P7-8 differed significantly from those at P17 and P28. There was a significant correlation between age and RI, consistent with an increasing contribution from Ca²⁺-impermeable (GluR2-containing) AMPARs during development, as seen in neocortical cells (Kumar et al. 2002). It has previously been shown that the single-channel conductance of AMPARs is high for Ca²⁺-permeable subtypes, relative to the Ca²⁺-impermeable (GluR2 containing) types (Swanson et al. 1997). We used peak-scaled non-stationary variance analysis of EPSCs (Traynelis et al. 1993) to compare the weighted mean channel-conductance at different ages. Estimates obtained from spontaneous EPSCs, demonstrated a decrease that correlated significantly both with RI and with age. In conclusion, our observations are consistent with the view that there is an increased contribution from GluR2-containing AMPARs, accompanied by a decrease in synaptic channel conductance during development of IN synapses.