GTP or G protein inactivation by N-ethylmaleimide reduced the Bmax value but not the KD value of 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT) binding in the rat subcortical limbic area. Neurotensin (10 nM) increased the KD and the Bmax values of [3H]7-OH-DPAT binding, and these effects persisted also following N-ethylmaleimide pretreatment. N-Propylnorapomorphine, quinpirole, raclopride, and remoxipride inhibited [3H]7-OH-DPAT binding with Ki values of 0.093, 1.97, 10.6, and 710 nM, respectively. These findings indicate that the D3 receptor is coupled to G proteins in the brain, and that neurotensin can modulate D3 agonist binding by a G protein-independent mechanism.