Administration of E. coli endotoxin (1 mg/kg i.v.) abolished the acid secretory response induced by a bolus injection of pentagastrin (100 micrograms/kg i.v.) in the continuously perfused stomach of the anaesthetized rat. Endotoxin administration did not modify mean systemic arterial blood pressure. Pretreatment with NG-nitro-L-arginine methyl ester (L-NAME; 5-20 mg/kg i.v.), but not dexamethasone (5 mg/kg s.c. twice) or indomethacin (5 mg/kg i.m.), substantially restored the secretory responses to pentagastrin. The actions of L-NAME were reversed by the prior administration of L-arginine (100 mg/kg i.v.), but not by its enantiomer D-arginine (100 mg/kg i.v.). L-NAME (10 mg/kg i.v.) increased blood pressure but this does not seem to be the mechanism by which endotoxin-induced acid inhibition was prevented, since similar systemic pressor responses induced by noradrenaline (15 micrograms/kg per min i.v.) had no such effect. The platelet-activating factor (PAF) receptor antagonist, WEB 2086 (2 mg/kg), induced a partial reversal of the inhibition by endotoxin of acid responses to pentagastrin. In endotoxin-treated rats, the combined administration of L-NAME (10 mg/kg) and WEB 2086 (2 mg/kg) completely restored the degree of H+ output induced by pentagastrin to levels similar to those of control, vehicle-treated animals. These findings suggest that endotoxin-induced acute inhibition of acid responses to pentagastrin involves NO synthesis and the release of PAF.