The protective effects of ultra-low doses (ULD) of glutamate against glutamate toxicity was studied in primary rat spinal, cortical and cerebellar neurons. Neurons were exposed to four subtoxic, ultra-low concentrations of glutamate (10(-18) M, 10(-20)M, 10(-22) M and 10(-30) M) for 72 h and then subsequently challenged with toxic concentrations (25 microM) of glutamate. Neuron viability was consistently 10% higher in spinal and cortical neurons pre-exposed to glutamate concentrations of 10(-18) M and 10(-22) M, and in cerebellar neurons pre-exposed to 10(-20) M and 10(-30) M. Using laser scanning confocal microscopy and the fluorescent calcium probe fluo-3, we found no alterations in intracellular calcium dynamics in the protected cells. This protective effect is consistent with a growing body of evidence for tolerance induced by low-dose toxin exposure but is the first time that such tolerance has been demonstrated with ultra-low glutamate exposure. Our data show that pre-exposure of neuronal cells to ULD glutamate can protect against subsequent exposure to toxic levels of glutamate.