Nicotine, acting at pentameric neuronal nicotinic acetylcholine receptors (nAChRs), is the primary addictive component in tobacco. At low doses, it affects attention, learning, memory, anxiety, cardiovascular responses, thermoregulation, and nociception. At high doses, nicotine produces more drastic behaviors and eventually induces tonic-clonic seizures in rodents. In mammals, several subunits of the nAChRs have been cloned, including eight α and three β subunits. To study the physiological role of the α5 subunit, we have generated α5-deficient mice. These mice have a generally healthy appearance and are normal in a standard battery of behavioral tests. However, the sensitivity of α5 mutant mice to nicotine-induced behaviors and seizures is dramatically reduced compared with their wild-type littermates. These animals have a normal brain anatomy and normal levels of mRNA for other nAChR subunits, namely α4, α6, α7, β2, and β4. In addition, 125I-epibatidine and [125I]αbungarotoxin binding in the brains of α5-deficient mice is normal. Together, these results suggest a direct involvement of the α5 subunit in the observed phenotypes.