Quantitative receptor autoradiography was performed to test the effects of neurotensin (NT)/neuromedin N (NN) peptides on dopamine (DA) D2 receptors in rat neostriatal sections. Competition experiments showed that 10 nM of NT, 10 nM of NN, and 1 nM of the C-terminal NT-(8-13) fragment produced a 2-fold increase in the IC50 value and a 20% decrease in the B0 value of DA for the D2 antagonist [125I]iodosulpride binding sites. The results demonstrate a stronger reduction in the affinity of DA for neostriatal D2 receptors by the NT/NN peptides in brain sections compared with membrane preparations indicating the possible involvement of cytoplasmic factors and/or a demand for a more intact membrane structure in these receptor-receptor interactions. Further evidence for the hypotheses that the C-terminal NT fragments may be among the major ligands for the neostriatal NT receptors and that the neostriatal NT receptors may be a new subtype of NT receptor has been obtained.