Cerebellar Purkinje neurons express the glutamate transporter, EAAT4, which is crtical for the removal of glutamate from excitatory synapses, thus maintaining normal neurotransmission by preventing synapse spill-over and glutamate mediated excitotoxicity. In addition, EAAT4 has a prominent glutamate-gated chloride conductance of unknown physiological relevance. Purkinje neurons fire action potentials (AP) tonically in the absence of synaptic activity, but this firing rate is modulated by excitatory and inhibitory synaptic inputs to produce the functional output. The influence of EAAT4 on intrinsic excitability of Purkinje neurons is unknown. Here we investigate the effects of EAAT4 reduction and absence on spontaneous firing activity of current-clamped Purkinje neurons using acute cerebellar slices from EAAT4-/- and EAAT4+/- mice (Huang et al, 2004). Mean spontaneous Purkinje neuron firing rates in EAAT4-/- and EAAT4+/- mice appeared higher than wildtype (WT) controls although this effect was not significant (WT, 34.6 ±1.7 Hz, n = 5; EAAT4+/-, 56.8 ± 9.5 Hz, n = 7; EAAT4-/-, 54.2 ± 10.1 Hz, n = 9, data are mean ± S.E.M). However, firing rates observed in EAAT4-/- and EAAT4+/- mice were variable, with some cells firing at similar levels to WT, whereas some showed much higher rates. Furthermore, a subset of Purkinje cells from both EAAT4-/- and EAAT4+/- mice displayed bursting patterns of spontaneous activity rather than the regular tonic firing always observed in WT mice (bursting observed in 0/5 WT, 4/9 EAAT4+/- and 5/15 EAAT4-/- cells). This variation in firing rate and bursting activity patterns could be related to the non-uniform expression of EAAT4 in the cerebellum, with higher and lower expression occurring in a banding pattern (Dehnes et al, 1998). In cells showing a bursting pattern of AP activity, blockade of voltage-gated sodium channels by tetrodotoxin (TTX) revealed a TTX-insensitive ‘spike’ not observed in WT cells, indicating it is not mediated by sodium currents but likely to be due to dendritic calcium channel activity. The data presented suggest wild type levels of EAAT4 are required for normal Purkinje cell intrinsic excitability. However, the ionic mechanisms underlying the bursting pattern as well as verification of a correlation between EAAT4 expression levels and spontaneous firing rate require further investigation.