Interleukin-1 beta (IL-1 beta) is the most potent inhibitor of gastric acid secretion known at present. Although histamine has been shown to be an important mediator of gastric acid secretion, the effect of IL-1 beta on gastric histamine mobilization has not been studied. In the present study, the effects of IL-1 beta on gastric acid secretion and gastric histamine mobilization were investigated in conscious rats with both gastric and vesical fistulas. IL-1 beta (5 micrograms/kg iv) significantly inhibited basal acid secretion but did not affect basal urinary histamine excretion and fundic histidine decarboxylase (HDC) activity. Gastrin-17-I (1 nmol.kg-1.h-1) caused a marked increase in acid secretion, urinary histamine secretion, and fundic HDC activity. IL-1 beta (5 micrograms/kg iv) completely inhibited gastrin-induced acid secretion and partially inhibited urinary histamine excretion and fundic HDC activity. Pretreatment with indomethacin (10 mg/kg ip) partially reversed the inhibitory effects of IL-1 beta on gastrin-stimulated fundic HDC activity and acid secretion. These findings indicate that IL-1 beta inhibits gastric histamine mobilization through both prostaglandin-dependent and prostaglandin-independent pathways. Furthermore, it is suggested that the inhibitory action of IL-1 beta on gastric acid secretion is mediated by the inhibition of gastric histamine mobilization.