PET studies were carried out on brain dopamine D1 receptors using two new ligands, [11C]SCH 39166 and [11C]NNC 756.Four normal subjects and eight predominantly unilateral patients with early Parkinson's disease were investigated. Each of them underwent both a PET scan with [11C]SCH 39166 and one with [11C]NNC 756. A dose of about 185 MBq (5 mCi) of these ligands was administered intravenously and a dynamic PET scan with an ECAT 931/08 PET camera was carried out. Ratios between the striatal and cerebellar uptake of these compounds were calculated.Both [11C]SCH 39166 and [11C]NNC 756 accumulated in the striatum. There was also some neocortical binding; 75% of the striatal value in the case of [11C]SCH 39166 and 60% with [11C]NNC 756 which displayed higher (p < 0.01) uptake in the striatum than [11C]SCH 39166. There were no significant side-to-side differences in the controls nor in the parkinsonian patients.These results imply that both [11C]SCH 39166 and [11C]NNC 756 can be used in PET studies for the visualization and quantification of dopamine D1 receptors. Since [11C]NNC 756 has a significantly better signal-to-noise ratio in the striatum than [11C]SCH 39166, it seems to offer definite advantages for studies of D1 receptors.