The role of cannabinoid (CB) receptors in the regulation of gastric acid secretion was investigated in the rat by means of functional experiments and by immunohistochemistry. 2 In anaesthetized rats with lumen-perfused stomach, the non selective CB-receptor agonist WIN 55,212-2 (0.30-4.00 mumol kg(-1), i.v.) and the selective CBI-receptor agonist HU-210 (0.03-1.50 mumol kg(-1), i.v.), dose-dependently decreased the acid secretion induced by both pentagastrin (30 nmol kg (-1) h(-1)) and 2-deoxy-D-glucose (1.25 mmol kg 1, i.v.). By contrast, neither WIN 55,2122 (1-4 mumol kg(-1), i.v.) nor HU-210 (0.03-1.50 mumol kg(-1), i.v.) did modify histamine-induced acid secretion (20 mumol kg(-1) h(-1)). The selective CB2-receptor agonist JWH-015 (3 - 10 mumol kg(-1), i.v.) was ineffective. 3 The gastric antisecretory effects of WIN 55,212-2 and HU-210 on pentagastrin-induced acid secretion were prevented by the selective CBI-receptor antagonist SR141716A (0.65 mumol kg(-1), i.v.) and unaffected by the selective CB2-receptor antagonist SR144528 (0.65-2 mumol kg(-1), i.v.). 4 Bilateral cervical vagotomy and ganglionic blockade with hexamethonium (10 mg kg(-1), i.v., followed by continuous infusion of 10 mg kg(-1) h(-1)) significantly reduced, but not abolished, the maximal inhibitory effect of HU-210 (0.3 mumol kg(-1), i.v.) on pentagastrin-induced acid secretion; by contrast, pretreatment with atropine (1 mg kg 1, i.v.) did not modify the antisecretory effect of HU-210. 5 Immunoreactivity to the CB, receptor was co-localized with that of the cholinergic marker choline acetyltransferase in neural elements innervating smooth muscle, mucosa and submucosal blood vessels of rat stomach fundus, corpus and antrum. In contrast, CB2 receptor-like immunoreactivity was not observed. 6 These results indicate that gastric antisecretory effects of cannabinoids in the rat are mediated by suppression of vagal drive to the stomach through activation of CB, receptors, located on pre- and postganglionic cholinergic pathways. However, the ineffectiveness of atropine in reducing the effect of HU-210 suggests that the release of non cholinergic excitatory neurotransmitters may be regulated by CB, receptors.