Recombinant cell lines expressing D2 and D3 dopamine receptor isoforms have been used to study coupling of these receptors to G-proteins from the effects of GTP to reduce agonist binding affinities in agonist/[3H]spiperone competition experiments. D2(short) expressed in Ltk- cells, D2(long) expressed in Ltk- or CHO cells and D3 expressed in CHO cells all showed coupling to the endogenous G-proteins of the cells. The detailed agonist binding characteristics indicated that there were differences in receptor/G-protein coupling for the same receptor(D2(long)) expressed in two cell types or for different receptors (D2(short), D2(long)) expressed in the same cell type.