Neuronal nicotinic acetylcholine receptors mediate nicotine's diverse effects on the brain, spinal cord and autonomic nervous system. These receptors are composed of alpha and beta subunits. Eight different alpha and three different beta subunits have been identified in vertebrate nervous systems, giving rise to the possibility of multiple subtypes of nicotinic receptors, defined by their constituent subunits. The pharmacological and channel conductance properties of the recombinant receptor subtypes studied in cellular expression systems differ from one another. In addition, the regulation of the receptor density and function during and after acute and chronic exposure to nicotine appears to differ among the subtypes. The predominant receptor subtypes in specific brain regions and peripheral neurons are beginning to be identified and their characteristics studied using new ligands and methods. As more is learned of the differences among the receptor subtypes, it should be possible to identify which specific subtype mediates a specific function within the nervous system and which subtypes are associated with the reinforcing and addictive actions of nicotine.