This chapter presents an overview of electrophysiology of neuronal nicotinic acetylcholine receptors (nAChR) expressed in Xenopus oocytes following nuclear injection of genes or cDNAs. The Xenopus oocyte system allows testing the functions of the many neuronal nAChR-related subunits isolated by molecular cloning. In vivo, some brain nuclei and peripheral nervous system ganglia contain many different species of nAChR-related subunit mRNAs and may, therefore, simultaneously express several neuronal nAChR subtypes. Moreover, there is molecular and physiological evidence that some of the neuronal subunits that have been isolated do not assemble into conventional nAChRs at all. Thus, injecting oocytes with defined pairs of cloned subunit cRNAs or cDNAs is the only direct and rapid way to determine which pairs of subunits assemble into functional receptors and which do not, and what the detailed electrophysiological properties of the various reconstituted receptors are.