[125I]Iodosulpride binding was studied in frontal rat brain sections by quantitative autoradiography. Using preincubated (= washed) sections, selective labelling and identification of dopamine D3 receptors was obtained using 0.2 nM [125I]iodosulpride in the presence of 100 nM domperidone for the occlusion of the D2 receptors. A high density of D3 receptors was noticed in the islands of Calleja. When preincubation of the sections was omitted, no D3 receptor labelling could be achieved, indicating tight binding to the receptor of an endogenous inhibitor. Such a tight receptor occupancy was not observed for the D2 receptor and various other neurotransmitter receptors. The occlusion of the D3 receptor could be prevented by tetrabenazine-induced monoamine depletion of the rats. It can be concluded, therefore, that D3 receptors are massively occupied by a monoamine, likely to be dopamine. This observation prompts the question to what extent dopamine D3 receptors can become occupied in vivo by systematically applied exogenous compounds.