Mice lacking the voltage-gated potassium channel α subunit, K_V1.1, display frequent spontaneous seizures throughout adult life. In hippocampal slices from homozygous K_V1.1 null animals, intrinsic passive properties of CA3 pyramidal cells are normal. However, antidromic action potentials are recruited at lower thresholds in K_V1.1 null slices. Furthermore, in a subset of slices, mossy fiber stimulation triggers synaptically mediated long-latency epileptiform burst discharges. These data indicate that loss of K_V1.1 from its normal localization in axons and terminals of the CA3 region results in increased excitability in the CA3 recurrent axon collateral system, perhaps contributing to the limbic and tonic–clonic components of the observed epileptic phenotype. Axonal action potential conduction was altered as well in the sciatic nerve—a deficit potentially related to the pathophysiology of episodic ataxia/myokymia, a disease associated with missense mutations of the human K_V1.1 gene.