A novel series of ligands with substitutions at the 5-position on phenyl ring A and at the 4'-position on phenyl ring B of 2-(2'-((dimethylamino)methyl)-4'-(fluoroalkoxy)phenylthio)benzenamine (4'-2-fluoroethoxy derivatives 28-31 and 4'-3-fluoropropoxy derivatives 40-42) were prepared and tested as serotonin transporter (SERT) imaging agents. The new ligands displayed high binding affinities to SERT (Ki ranging from 0.03 to 1.4 nM). The corresponding 18F labeled compounds, which can be prepared readily, showed excellent brain uptake and retention after iv injection in rats. The hypothalamus region showed high uptake values between 0.74% and 2.2% dose/g at 120 min after iv injection. Significantly, the hypothalamus to cerebellum ratios (target to nontarget ratios) at 120 min were 7.8 and 7.7 for [18F]28 and [18F]40, respectively. The selective uptake and retention in the hypothalamus, which has a high concentration of SERT binding sites, demonstrated that [18F]28 and [18F]40 are promising positron emission computed tomography imaging agents for mapping SERT binding sites in the brain.