To elucidate the mechanisms of flavonoid-induced protection against nonsteroidal anti-inflammatory drug (indomethacin)-induced acute gastric damage, the effects of 5-methoxyflavone and 5-methoxyflavanone on the gastric vasculature were compared both in vivo (using laser Doppler flowmetry in anesthetized rats) and in vitro on rat superior mesenteric arteries. The effects of the compounds on indomethacin-induced leukocyte adherence to mesenteric venules were investigated by intravital videomicroscopy. Oral 5-methoxyflavone reduced indomethacin-induced macroscopic damage by 38 to 99% (ED50 = 5.5 mg/kg). Damage was not significantly reduced by 5-methoxyflavanone. Light microscopy studies also demonstrated a reduction in damage severity. 5-Methoxyflavone, but not 5-methoxyflavanone, increased the gastric conductance significantly. The effects on isolated mesenteric arteries correlated with the effects on in vivo conductance. Finally, indomethacin-induced leukocyte adherence was inhibited to a greater extent by 5-methoxyflavone than by 5-methoxyflavanone. In conclusion, the flavonoid 5-methoxyflavone provides gastroprotection against nonsteroidal anti-inflammatory drug-induced gastric damage. A structurally similar compound, 5-methoxyflavanone, demonstrated minimal gastroprotective activity, suggesting that the double bond of 5-methoxyflavone is required for biological activity. The finding that 5-methoxyflavone (but not 5-methoxyflavanone) significantly increased gastric vascular perfusion and reduced leukocyte adherence to mesenteric venules suggests that these mechanisms may contribute to the flavonoid's gastroprotective activity.