The P/Q type voltage-gated Ca²⁺ channels are involved in membrane excitability and Ca²⁺-dependent neurotransmitter release within the CNS. Mutations in the Cacna1A gene encoding the α1A subunit of the P/Q type Ca²⁺ channel have recently been reported in tottering mice and a more severely affected allele, leaner. Here we show using in vivo cortical microdialysis that evoked increases of extracellular glutamate levels are markedly attenuated in both mutants upon KCl-induced depolarization compared with wild-type mice. Tottering and leaner mice also show a 10-fold resistance to cortical spreading depression induced by cortical electrical stimulation or KCl application to the pial surface. A slower transcortical propagation speed and failure to sustain regenerative spread of the depolarizing wave were more pronounced in leaner neocortex. Both signaling defects appeared unrelated to the developmental history of repeated cortical spike-wave discharges, since neither were observed in the stargazer mouse, a Ca²⁺ channel γ2 subunit mutant with a similar seizure phenotype. These data demonstrate two cortical excitability defects revealed by prolonged depolarization in cerebral networks expressing mutant P/Q type Ca²⁺ channels, and are the first to identify a gene linked to a spreading depression phenotype.