The ability of leukotrienes derived from eicosapentaenoic acid were compared with counterpart leukotrienes derived from arachidonic acid in terms of their ability to affect susceptibility of the stomach to injury induced by a topical irritant and their ability to alter gastric blood flow. Intra-arterial infusion of leukotriene C4 (LTC4) and LTD4 (0.1-3 micrograms/kg/min for 5 min) produced dose-dependent increases in gastric mucosal damage induced by topically applied 20% ethanol, as assessed macroscopically, by changes in transmucosal potential difference and by measurement of efflux of protein into the gastric lumen. Similar doses of LTC5 or LTD5 did not produce significant changes in any of these three parameters, when compared with control rats receiving the vehicle. With a higher dose of LTC5 or LTD5 (5 micrograms/kg/min), significant damage was observed. LTC4 and LTD4 were also found to be more potent at reducing gastric blood flow than LTC5 and LTD5. These results demonstrate that the peptido-leukotrienes derived from eicosapentaenoic acid (LTC5 and LTD5) are on the order of five times less potent than the leukotrienes derived from arachidonic acid (LTC4 and LTD4), in terms of increasing the susceptibility of the gastric mucosa to damage and reducing gastric blood flow. These results may have important implications in terms of the hypothesis that fish oil diets may be protective or may accelerate healing in ulcerative diseases of the gastrointestinal tract.