A series of 2-[(2-aminoethyl)thio]quinolines substituted at the 3-position with alkyl, aryl, or heteroaryl groups has been prepared in the search for novel and selective 5-HT2 antagonists. The affinity of the compounds for 5-HT1 receptor sites was measured by their ability to displace [3H]-5-HT from rat brain synaptosomes whereas the affinity for 5-HT2 receptor sites was measured by their ability to displace [3H]spiperone from synaptosomes prepared from rat brain cortex. The 5-HT2 antagonist properties of the compounds were measured in vivo by their antagonism of 5-hydroxytryptophan-induced head twitches in the mouse and by their antagonism of hyperthermia induced by fenfluramine (N-ethyl-alpha-methyl-m-(trifluoromethyl)phenethylamine hydrochloride) in the rat. The structure-activity relationships in this series are discussed and the properties of 2-[[2-(dimethylamino)ethyl]thio]-3-phenylquinoline hydrochloride (70) are highlighted.