Abstract The striatum contains a high density of histamine H 3 receptors, but their role in striatal function is poorly understood. Previous studies have demonstrated antagonistic interactions between striatal H 3 and dopamine D 1 receptors at the biochemical level, while contradictory results have been reported about interactions between striatal H 3 and dopamine D 2 receptors. In this study, by using reserpinized mice, we demonstrate the existence of behaviorally significant antagonistic postsynaptic interactions between H 3 and D 1 and also between H 3 and dopamine D 2 receptors. The selective H 3 receptor agonist imetit inhibited, while the H 3 receptor antagonist thioperamide potentiated locomotor activation induced by either the D 1 receptor agonist SKF 38393 or the D 2 receptor agonist quinpirole. High scores of locomotor activity were obtained with H 3 receptor blockade plus D 1 and D 2 receptor co-activation, i.e., when thioperamide was co-administered with both SKF 38393 and quinpirole. Radioligand binding experiments in striatal membrane preparations showed the existence of a strong and selective H 3–D 2 receptor interaction at the membrane level. In agonist/antagonist competition experiments, stimulation of H 3 receptors with several H 3 receptor agonists significantly decreased the affinity of D 2 receptors for the agonist. This kind of intramembrane receptor–receptor interactions are a common biochemical property of receptor heteromers. In fact, by using Bioluminescence Resonance Energy Transfer techniques in co-transfected HEK-293 cells, H 3 (but not H 4) receptors were found to form heteromers with D 2 receptors. This study demonstrates an important role of postsynaptic H 3 receptors in the modulation of dopaminergic transmission by means of a negative modulation of D 2 receptor function.