We report here the regulation of agonist interactions with [3H]SCH23390-labeled D1 dopamine receptors in rat striatum. Scatchard analyses of [3H]SCH23390 saturation data revealed a single high affinity binding site (KD = 0.49 nM) with a Bmax of 64 pmol/g tissue. The specific binding of 0.25 nM [3H]SCH23390 represented 90% of total binding. Antagonist competition for [3H]SCH23390 binding was monophasic (i.e. pseudo-Hill slope approximately 1) and the rank order of antagonists' affinities was consistent with the pharmacology of D1 dopamine receptors (e.g. cis-flupentixol greater than haloperidol greater than spiperone). In contrast, agonist competition curves were shallow (pseudo-Hill slope less than 1) and computer-assisted analysis indicated that, for all agonists, the data best fit a two-site model composed of a high (KH) and a low (KL) affinity component. In the presence of 0.3 mM GTP, the high affinity binding component (%RH) of various agonists was reduced by approximately 50%. No significant effect of 0.3 mM GTP on [3H]SCH23390 binding was observed. Additionally, it was noted that [3H]SCH23390 labels S2 serotonin receptors in extrastriatal brain regions. However, [3H]SCH23390 apparently does not have an affinity high enough to label S2 receptors at the concentration of [3H]SCH23390 employed in labeling striatal D1 dopamine receptors. These data indicate that [3H]SCH23390 represents a superior radioligand for labeling the two-state striatal D1 dopamine receptor in that its high percent specific binding makes it especially suitable for detailed mechanistic studies of this receptor.