In a mouse mutagenesis screen, we isolated a mutant, Myshkin ( Myk ), with autosomal dominant complex partial and secondarily generalized seizures, a greatly reduced threshold for hippocampal seizures in vitro, posttetanic hyperexcitability of the CA3-CA1 hippocampal pathway, and neuronal degeneration in the hippocampus. Positional cloning and functional analysis revealed that Myk /+ mice carry a mutation (I810N) which renders the normally expressed Na ⁺ ,K ⁺ -ATPase α3 isoform inactive. Total Na ⁺ ,K ⁺ -ATPase activity was reduced by 42% in Myk /+ brain. The epilepsy in Myk /+ mice and in vitro hyperexcitability could be prevented by delivery of additional copies of wild-type Na ⁺ ,K ⁺ -ATPase α3 by transgenesis, which also rescued Na ⁺ ,K ⁺ -ATPase activity. Our findings reveal the functional significance of the Na ⁺ ,K ⁺ -ATPase α3 isoform in the control of epileptiform activity and seizure behavior.