The Shaw-like potassium (K(+)) channels Kv3.1 and Kv3.3 are widely coexpressed in distinct neuronal populations in the CNS, possibly explaining the relatively "mild" phenotypes of the Kv3.1 and the Kv3.3 single mutant. Kv3.1-deficient mice show increased cortical gamma- and decreased delta-oscillations (Joho et al., 1997, 1999); otherwise, the Kv3.1-mutant phenotype is relatively subtle (Ho et al., 1997; Sánchez et al., 2000). Kv3.3-deficient mice display no overt phenotype (Chan, 1997). To investigate whether Kv3.1 and Kv3.3 K(+) channels are functionally redundant, we generated the Kv3.1/Kv3.3 double mutant. Kv3.1/Kv3.3-deficient mice were born at the expected Mendelian frequencies indicating that neither Kv3.1 nor Kv3.3 K(+) channels are essential for embryonic development. Although there are no obvious changes in gross brain anatomy, adult Kv3.1/Kv3.3-deficient mice display severe ataxia, tremulous movements, myoclonus, and hypersensitivity to ethanol. Mice appear unbalanced when moving, whereas at rest they exhibit whole-body jerks every few seconds. In spite of the severe motor impairment, Kv3.1/Kv3.3-deficient mice are hyperactive, show increased exploratory activity, and display no obvious learning or memory deficit. Myoclonus, tremor, and ethanol hypersensitivity are only seen in the double-homozygous Kv3.1/Kv3.3-deficient mice, whereas increased locomotor and exploratory activity are also present in double-heterozygous mice. The graded penetrance of mutant traits appears to depend on the number of null alleles, suggesting that some of the distinct phenotypic traits visible in the absence of Kv3.1 and Kv3.3 K(+) channels are unrelated and may be caused by localized dysfunction in different brain regions.