•  Cerebellar ataxias are progressive debilitating diseases with no known treatment and are associated with defective motor function and, in particular, abnormalities to Purkinje cells. •  Mutant mice with deficits in Ca2+ channel auxiliary α2δ‐2 subunits are used as models of cerebellar ataxia. •  Our data in the du2J mouse model shows an association between the ataxic phenotype exhibited by homozygous du2J/du2J mice and increased irregularity of Purkinje cell firing. •  We show that both heterozygous +/du2J and homozygous du2J/du2J mice completely lack the strong presynaptic modulation of neuronal firing by cannabinoid CB1 receptors which is exhibited by litter‐matched control mice. •  These results show that the du2J ataxia model is associated with deficits in CB1 receptor signalling in the cerebellar cortex, putatively linked with compromised Ca2+ channel activity due to reduced α2δ‐2 subunit expression. Knowledge of such deficits may help design therapeutic agents to combat ataxias. Abstract  Cerebellar ataxias are a group of progressive, debilitating diseases often associated with abnormal Purkinje cell (PC) firing and/or degeneration. Many animal models of cerebellar ataxia display abnormalities in Ca2+ channel function. The ‘ducky’du2J mouse model of ataxia and absence epilepsy represents a clean knock‐out of the auxiliary Ca2+ channel subunit α2δ‐2, and has been associated with deficient Ca2+ channel function in the cerebellar cortex. Here, we investigate effects of du2J mutation on PC layer (PCL) and granule cell layer (GCL) neuronal spiking activity and, also, inhibitory neurotransmission at interneurone–Purkinje cell (IN‐PC) synapses. Increased neuronal firing irregularity was seen in the PCL and, to a less marked extent, in the GCL in du2J/du2J, but not +/du2J, mice; these data suggest that the ataxic phenotype is associated with lack of precision of PC firing, that may also impinge on GC activity and requires expression of two du2J alleles to manifest fully. The du2J mutation had no clear effect on spontaneous inhibitory postsynaptic current (sIPSC) frequency at IN‐PC synapses, but was associated with increased sIPSC amplitudes. du2J mutation ablated cannabinoid CB1 receptor (CB1R)‐mediated modulation of spontaneous neuronal spike firing and CB1R‐mediated presynaptic inhibition of synaptic transmission at IN‐PC synapses in both +/du2J and du2J/du2J mutants, effects that occurred in the absence of changes in CB1R expression. These results demonstrate that the du2J ataxia model is associated with deficient CB1R signalling in the cerebellar cortex, putatively linked with compromised Ca2+ channel activity and the ataxic phenotype.