An intron-containing gene encoding a novel human serotonin (5-HT) receptor was isolated from human genomic and cDNA libraries with probes directed to transmembrane regions of the adenylate cyclase stimulatory Drosophila serotonin receptor gene, 5-HTdrol. Membranes harvested from transiently transfected Cos-7 cells displayed high affinity (Kd = 8.5 nM), saturable (Bmax = 6.6 pmol/mg protein) [3H]5-HT binding. The rank order of potencies for serotonergic ligands to displace specific [3H]5-HT binding was: 5-carboxamido-tryptamine > methiothepin > metergoline > 5-HT > 8-hydroxy-2-(di-n-propylamino)tetralin > sumatriptan > ketanserin > zacopride. 5-HT produced a dose-dependent (EC50 = 992 nM) stimulation (approximately 20-fold) of cAMP accumulation in transiently transfected cells, and this response was antagonized by the nonselective 5-HT antagonist methiothepin. RNA for this gene was predominantly detected in the human brain and a subset of peripheral tissues including coronary artery and several tissues of the gastrointestinal tract. The molecular biological and pharmacological properties of this receptor suggest that it is the first member of a new serotonin receptor subfamily (5-HT7). The second messenger coupling, and tissue distribution indicate a possible identity to 5-HT receptors that mediate relaxant responses in certain isolated blood vessels.