The new selective D1-dopamine receptor antagonist SCH 39166 was labelled with the positron emitting isotope 11C and used as ligand for visualization of dopamine-D1 receptor binding in Cynomolgus monkeys by PET. After intravenous administration of the ligand a marked uptake of radioactivity was recorded in the D1-dopamine receptor-rich striatum and neocortex but not in the dopamine receptor-poor cerebellum. The uptake of radioactivity in striatum and neocortex was markedly displaced after the intravenous injection of a high dose of the D1-dopamine receptor antagonist SCH 23390 but not after the 5-HT2 receptor antagonist ketanserine. 11C-SCH 39166 should be a useful tool to explore D1-dopamine receptor characteristics in the living human brain by PET.