3H-SCH-23390, a selective antagonist of D-1 dopamine (DA) receptors, was used in a radioreceptor assay with rat brain striatal tissue, optimized biochemically, and extensively characterized pharmacologically with striatal membranes. Nonspecific binding, defined with excess cis(Z)-flupenthixol (300 nM), averaged 20-25% of total counts bound. Specific binding was linearly dependent on the amount of original striatal tissue (0-4 mg) or protein (0-250 micrograms), temperature dependent, saturable and reversible, and appeared to involve a single site at ligand concentrations limited to less than 10 nM. Binding in rat brain regions ranked as: striatum greater than accumbens greater than prefrontal cortex greater than posterior cerebral cortex greater than cerebellum. Association was virtually complete within 30 min at 30 degrees, and the rate of dissociation at 30 degrees was 0.0377 min-1 (half-time = 18.4 min). Affinity (Ka or Kd) determined from association and dissociation rate constants and from concentration isotherms averaged 0.349 and 0.340 nM respectively. Including Na+ at 150 mM increased apparent maximum specific binding (Bmax) by 19%, with a 29% increase in affinity; other monovalent cations alone had small effects on specific binding; Ca2+ and Mg2+ reduced binding by 42%. Agents (N = 85) were tested for potency (Ki or IC50) in competition with the ligand (at 0.30 nM). Those known to have selective effects at D-1 receptors, generally, were most potent and stereoselective. Na+ (150 mM) had little effect on the affinity of cis-thioxanthenes but decreased that of most other agents tested with high D-1 affinity. For antipsychotic agents, the correlation of typical clinical daily doses versus Ki at D-1 sites (r = 0.06) was much lower than at D-2 sites (r = 0.94). (-)Thioridazine was discovered to be D-1 selective, whereas the (+) enantiomer was selective for D-2 sites labeled with 3H-spiperone. Relatively sedating antidepressants had greater D-1 affinity than their less-sedating, secondary amine congeners.