[125I]SCH 23982, [125I]8-iodo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol, binds reversibly and with high affinity (Kd = 0.7 +/- 0.05 nM) to specific binding sites (maximum binding = 108 +/- 3.5 fmol/mg) in the caudate nucleus of the rat brain. The caudate binding site displays pharmacological properties similar to the D-1 receptor; furthermore, the site has a low affinity for drugs favoring the D-2 receptor. GTP diminishes the affinity of dopamine, a D-1 agonist, but not SCH 23390, a D-1 antagonist, for the caudate binding site. The iodinated ligand discriminates between the D-1 and the D-2 dopamine receptors: the neurointermediate lobe of the rat pituitary gland, a tissue rich in D-2 dopamine receptors, possesses less than 4% of the specific binding sites in the caudate. Because [125I]SCH 23982 exists at a higher specific activity than [3H]SCH 23390, a ligand used previously to identify the D-1 receptor, and because the results obtained with the iodinated ligand are otherwise similar to those obtained with the [3H]SCH 23390, [125I]SCH 23982 is the ligand of choice for identifying the D-1 receptor.