A series of new, fluorine-containing substituted diphenyl sulfides was synthesized to serve as candidate ligands for positron emission tomography (PET) imaging of the serotonin transporter (SERT) and to further probe the structure-activity relationship (SAR) of this class of compounds. Candidate compounds were assayed for their affinities to the monoamine transporters (SERT, norepinephrine transporter (NET), and dopamine transporter (DAT)) in competitive binding experiments in vitro using cloned human transporters. From these in vitro assays, four compounds (7c-f) were chosen for further evaluation. All four compounds have nanomolar affinity for SERT (K(i) 1.46 nM, 1.04 nM,1.83 nM, and 3.58 nM for 7c, 7d, 7e, and 7f, respectively). The F-18-labeled compounds, 16 and 18a-c, were prepared via a two-step radiosynthesis. Biodistribution studies in rats indicated that the F-18-labeled compounds localized in brain regions with high concentrations of SERT. Furthermore, competition experiments demonstrated that the binding of these radioligands in the rat brain was saturable, specific, and selective to SERT. Specific binding in the rat hypothalamus peaked at 5.6 for ligand 16 and 4.4 for 18b at 90 min after radioactivity administration. For ligand 18a, this same ratio was 8.4 at 120 min postinjection, while compound 18c displayed a lower specific binding ratio of 2.4. In summary, four F-18-labeled ligands were prepared and evaluated as candidate PET imaging agents for SERT. Among these four ligands, three appear to be promising radioligands suitable for the labeling of SERT in vivo, with 18a providing a higher specific binding in vivo than 16 or 18b.