The effect of cysteamine on gastric blood flow and on the indomethacin-induced gastric mucosal damage was studied. In anesthetized rats, cysteamine (280 mg/kg) given subcutaneously (s.c.) decreased gastric blood flow measured by the laser Doppler flowmetry technique. In contrast, cysteamine (1-60 mg/ml) applied topically to the serosal surface of the stomach evoked a concentration-dependent and long-lasting increase in gastric blood flow. At 60 mg/ml, cysteamine increased blood flow by 166.8 +/- 26.1% of predrug control value. Pretreatment with indomethacin (20 mg/kg, s.c.), intravenous (i.v.) atropine (1 mg/kg), propranolol (1 mg/kg, i.v.), combined H1 and H2-blockade or bilateral cervical vagotomy alone or combined with i.v. guanethidine (8 mg/kg), or pretreatment with the capsaicin analogue resiniferatoxin did not reduce the vasodilator response to cysteamine. The vasodilator response to topical capsaicin, was not reduced after s.c. cysteamine (280 mg/kg) pretreatment. In conscious pylonus-ligated rats, s.c. cysteamine (100 or 280 mg/kg) given simultaneously with indomethacin inhibited gastric acid output but had variable effects on the indomethacin-induced gastric mucosal damage. Cysteamine (100 or 280 mg/kg) administered s.c. 4 h prior to indomethacin enhanced gastric injury by s.c. indomethacin, but did not prevent the gastroprotective action of capsaicin. In contrast, orally administered cysteamine (60 mg/ml) reduced gastric injury induced by s.c. indomethacin plus intragastric HCl. These data provide the first evidence for the effect of cysteamine on gastric microcirculation in the rat and suggest a direct vasodilator effect for topical cysteamine. The microvascular effects of cysteamine are largely responsible for the different effects of this agent on experimental gastric injury.