6-Thiolato-substituted 2-(4'- N,N-dimethylamino)phenylimidazo[1,2- a]pyridines ( RS-IMPYs; 1- 4) were synthesized as candidates for labeling with carbon-11 ( t 1/2 = 20.4 min) and imaging of A beta plaques in living human brain using positron emission tomography (PET). K i values for binding of these ligands to Alzheimer's disease brain homogenates were measured in vitro against tritium-labeled 6 (Pittsburgh compound B). MeS-IMPY ( 3, K i = 7.93 nM) was labeled with carbon-11 at its S- or N-methyl position to give [ (11)C] 7 or [ (11)C] 8, respectively. After injection into rats, [ (11)C] 7 or [ (11)C] 8 gave moderately high brain uptakes of radioactivity followed by rapid washout to low levels. The ratio of radioactivity at maximal uptake to that at 60 min reached 18.7 for [ (11)C] 7. [ (11)C] 7 behaved similarly in mouse and monkey. [ (11)C] 7 also bound selectively to A beta plaques in post mortem human Alzheimer's disease brain. Although rapidly metabolized in rat by N-demethylation, [ (11)C] 7 was stable in rat brain homogenates. The ex vivo brain radiometabolites observed in rats have a peripheral origin. Overall, [ (11)C] 7 merits further evaluation in human subjects.