AbstractThe11C‐labelled radioligands raclopride, SCH 23390 and N‐methyl‐spiroperidol are well established for examination of central dopamine receptors by positron emission tomography (PET).Thin layer chromatography was used to examine the composition of radioactivity in plasma after intravenous injection of any of these three ligands into human subjects. For all three ligands there was a considerable metabolism during the time of a PET‐experiment but to a different degree. Forty‐two minutes after injection (11C)raclopride was unchanged to 76 per cent, (11C)N‐methylspiroperidol to 57 per cent and (11C) SCH 23390 only to 13 per cent.A time curve for ligand metabolism is necessary for compartmental analysis with an arterial input function. The considerable ligand metabolism demonstrated shortly after i.v. injection motivates further the identification of main metabolites to evaluate if they pass the blood‐brain barrier and bind to
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