Chronic nicotine exposure in tobacco smokers or experimental animals is known to cause an increase in brain binding sites for nicotine. It has been proposed that this is an adaptive response of neurons to accumulation of chronically desensitized receptors. Acetylcholine receptors of the same (alpha 4)2(beta 2)3 subunit composition as the predominant subtype of brain nicotinic receptors with high affinity for nicotine have been expressed in Xenopus oocytes and in a permanently transfected fibroblast cell line. Chronic exposure of these cells to nicotine or another agonist is shown to result in an increase in receptor amount, indicating that nicotine-induced up-regulation reflects properties of the alpha 4 beta 2 receptor protein, rather than being an adaptive response unique to the neurons in which these receptors are normally expressed. The nicotine concentration dependence, time course, and extent of receptor up-regulation are similar to those reported for receptors in brain. Up-regulation does not appear to require ion flow through the ion channel, because it is also caused by mecamylamine, which blocks the ion channel, and because after prolonged exposure to nicotine most receptors become permanently unable to open their channels in response to nicotine binding. The noncompetitive antagonist mecamylamine blocks open channels more effectively, and so it is more effective at blocking channels in the presence of nicotine. Mecamylamine and nicotine are also synergistic in causing receptor up-regulation. Ligands that cause up-regulation appear to induce a conformation of the receptor that is removed from the surface and degraded more slowly.