SCH-23390 is a high-affinity antagonist selective for D1 dopamine receptors (Ki = 2.5 nM). It does not contain a functional group that can be conveniently coupled to commercially available resins for affinity chromatography or to prepare photolabels for photoaffinity labeling of receptors. To construct an affinity resin for purification of dopamine D1 receptors, an aldehyde analogue of SCH-23390, (+/-)-7-chloro-8-hydroxy-1-(4'-formylphenyl)-3-methyl-2,3,4,5-tetrahydro -1H- 3-benzazepine (ASCH), was synthesized. 8-Methoxy-1-(4'-bromophenyl)-SCH-23390 was lithiated, formylated, and O-demethylated to form the aldehyde. NMR and IR analyses were performed to characterize the product. Assays were performed with the radioligand [125I]SCH-23982 to define the biological activity of the aldehyde. ASCH displaced [125I]SCH-23982 binding from caudate membranes with a Ki value of 7.1 nM. ASCH has been coupled through the aldehyde group on the phenyl ring to diaminodipropylamine-agarose for affinity chromatography. After solubilization of caudate membranes in 1% digitonin, the affinity resin retained binding sites for [125I]SCH-23982 that were eluted with 10 mM SCH-23390. The aldehyde was also covalently coupled to biotin hydrazide for fluorescence labeling of dopamine D1 receptors. The biotin-conjugated aldehyde of SCH-23390 displaced [125I]SCH-23982 binding from caudate membranes with a Ki value of 9.3 nM.