The 5-HT2C receptor gene is unique among the members of the 5-HT receptor family by virtue of its genomic organisation. The human 5-HT2C receptor gene, unlike many other genes for guanine nucleotide binding (G)-proteins, contains three introns which interrupt the coding sequence into four exons. The first two introns are at equivalent positions as compared to the intervening sequences previously found in the 5-HT2(A) receptor gene, suggesting a close evolutionary relationship between both genes. Southern blot analysis shows that the 5-HT2C receptor gene is a single copy gene. Furthermore, we report the functional expression of a complementary DNA for the 5-HT2C receptor, cloned from hippocampal RNA. Membranes prepared from NIH 3T3 cells stably expressing the 5-HT2C receptor cDNA, displayed a single population of high affinity sites for the antagonist [3H]mesulergine (Kd = 2.9 +/- 0.4 nM, Bmax = 44.3 +/- 7.2 pmol/mg protein) as well as for [3H]5-HT (Kd = 9.9 +/- 0.7 nM, Bmax = 13.6 +/- 1.0 pmol/mg protein). Displacement of [3H]mesulergine and [3H]5HT binding by ligands indicated a pharmacological similarity of these binding sites with porcine and rat choroid plexus 5-HT2C receptors. Furthermore, activation of the 5-HT2C receptor with 5-HT results in an increased phospholipase C activity.