The dopamine D1 antagonist SCH 39166 was labelled with tritium and used for in vitro binding and autoradiography using human post mortem brain tissue. Competition studies on tissue from human nucleus caudatus showed that SCH 23390 inhibited the binding of [3H]SCH 39166 biphasically. The non-specific binding of [3H]SCH 39166 in both nucleus caudatus and cerebellum was lower after the addition of SCH 23390 or SCH 39166 than after flupentixol (10 microM). Autoradiography showed specific [3H]SCH 39166 binding in the caudate nucleus and putamen in the brain sections. The binding of [3H]SCH 39166 in the medial part of the caudate nucleus was very dense and similar to that obtained with [3H]SCH 23390, which was used as a reference ligand. Dense binding of [3H]SCH 39166 was also found in cortical regions, and binding was also obtained in the cerebellum and in the hippocampus. Addition of flupentixol (10 microM) abolished some but not all the binding of [3H]SCH 39166. The binding of [3H]SCH 39166 to caudate and putamen was not totally abolished by the addition of excess SCH 23390, while excess SCH 39166 diminished the binding of [3H]SCH 23390 in all regions. The present study indicates that [3H]SCH 39166, similar to [3H]SCH 23390, binds to dopamine D1 receptors in the human brain. It is concluded that [3H]SCH 39166 has a slightly different binding pattern than [3H]SCH 23390, which can be due to labelling of one or two additional binding site(s) pharmacologically unrelated to dopamine D1 receptors.