Monoiodo-[125I-Tyr3]neurotensin (NT) bound to a high affinity, low capacity binding component and a lower affinity, high capacity component in rat brain synaptic membranes. The antihistamine H1 agent levocabastine, which bears no structural relationship to NT, selectively and totally inhibited NT binding to its low affinity binding sites. The IC50 for levocabastine was 7 nM. Lowering the temperature of the binding assay from 25 to 4 degrees C markedly reduced the affinity of the high affinity NT binding site but did not affect the ability of levocabastine to discriminate between high and low affinity NT binding sites in rat brain membranes and tissue sections. Radioautographic studies of [125I-Tyr3]NT binding to rat brain tissue sections in the absence and presence of levocabastine revealed markedly different regional distributions of the two NT binding components. The levocabastine-sensitive NT binding site was present in membranes from rat and mouse brain but absent from rabbit brain membranes and from human brain tissue sections. It was also absent from mouse neuroblastoma N1E115 and human colonic adenocarcinoma HT29 cell membranes, two cell lines which have previously been shown to possess NT receptors functionally coupled to intracellular second messenger-generating systems. These findings are discussed in the light of the known properties of the high and low affinity NT binding sites in rat brain.