The effects of dopamine and guanine nucleotides on the binding of the D1 dopamine receptor antagonist ligand [3H]SCH 23390 were examined in membranes prepared from putamen, caudate and nucleus accumbens of human postmortem brain. Dopamine induced a concentration-dependent decrease in the apparent maximum number of binding sites (Bmax) in each brain region studied, and displaced binding in a biphasic manner consistent with the presence of both high and low affinity states of the D1 receptor; the GTP analogue Gpp(NH)p transformed this biphasic displacement to a monophasic pattern consistent with a shift of high affinity sites to a low affinity state. However, the selective D2 antagonist eticlopride did not reverse the action of dopamine to decrease Bmax. These data suggest that dopamine decreases Bmax for D1 receptors through a high affinity, guanine nucleotide-sensitive agonist binding site, but fail to reveal D1:D2 interactions at this synaptic level.