Histamine H1 receptors were visualized in the living dog brain using [11C]pyrilamine or [11C]doxepin by positron emission tomography (PET). The regional distribution of these carbon-11 labeled compounds in the brain corresponded well with that of the histamine H1 receptors separately determined by in vitro binding assay. The radioactivity in the brain was reduced by treatment with triprolidine (1 mg/kg), a histamine H1 antagonist. The results of our study indicate that it is feasible to visualize histamine H1 receptors in human brain using these 11C-labeled compounds and PET.