[en] A high performance liquid chromatographic system was developed for separation of ¹¹C-labeled 5-hydroxy-L-tryptophan ([¹¹C]HTP) and in vivo formed radiolabeled metabolites in rat brain tissue. Analysis of brain homogenate revealed that the main part of the radioactivity was associated with ¹¹C-labeled 5-hydroxyindole-3-acetic acid after intravenous injection of [¹¹C]HTP to the rat. The serotonin synthesis rate in the brain was calculated and closely correlated to the serotonin synthesis rate in monkey and human measured using positron emission tomography