Abstract: The human D4 dopamine receptor has been expressed in Sf9 insect cells where it appears to couple to endogenous G proteins. Increased guanine nucleotide exchange to G proteins is a reflection of receptor activation and can be followed using a [35S]GTPγS binding assay. By measuring D4 receptor stimulation of [35S]-GTPγS binding we have been able to characterize several dopaminergic compounds for their functional activity at this receptor. In Sf9 cells expressing the D4 receptor, dopamine, quinpirole, and dp-2-aminodihydroxy-1,2,3,4-tetrahydronaphthalene were all full agonists, whereas (−)-apomorphine appeared to be a partial agonist. No increase in [35S]GTPγS binding was observed for noninfected cells or cells infected with an unrelated sequence. The quinpirole-stimulated [35S]GTPγS binding could be inhibited by the antagonists clozapine, eticlopride, and haloperidol, and a Schild analysis of these data showed that all three compounds were acting as competitive antagonists of D4 receptors. The rank order of affinities derived from the Schild analysis correlated with that obtained from [3H]spiperone competition binding assays. In conclusion, we have shown that, using this assay system, it is possible to investigate functionally the pharmacology of a recombinant G protein-coupled receptor in the absence of any information regarding the eventual second messenger pathways involved.