We investigated the effect of 24 h sustained treatment with the mGlu1 receptor antagonist CPCCOEt on mGlu1 receptor signalling in primary cultures of rat cerebellar granule cells. In the absence of ionotropic glutamate (iGlu) blockers, the maximal inositol phosphate (IP) response (Emax) but not the potency of glutamate was significantly increased when cells were pre-exposed for 24 h with CPCCOEt. When the contribution of iGlu receptors to the glutamate-induced IP response was eliminated with the use of DNQX, the Emax was again increased but also the concentration eliciting 50% of the maximal glutamate stimulus was significantly decreased. In the absence of iGlu receptor inhibitors, the Emax of quisqualate, which likely mediates IP accumulation only via the mGlu1 receptor, was significantly increased in CPCCOEt-pretreated cells. Also, less quisqualate was needed to reach the same IP effect. The potency of R193845, a selective mGlu1 receptor antagonist, was significantly decreased in antagonist-pretreated cells. These findings demonstrate that 24 h sustained antagonist treatment can render mGlu1 receptors in neurons supersensitive to agonists, with a concomitant decrease in the effectiveness of antagonists.