The protective capability of Maalox against indomethacin-induced gastric damage was evaluated in hamsters. The effect of acidification on the gastroprotection provided by Maalox against such damage was also determined. Maalox was ineffective against indomethacin-induced gastric antral ulceration in hamsters. Acidification of this antacid to pHs of 1.5–3.5 resulted in significant (80–90%) gastroprotection against indomethacin. Macroscopic and histologic evidence of binding by acidified Maalox to the hamster antral mucosa was clearly evident. In summary, no correlation exists between acid neutralization and the gastroprotective capability of Maalox against indomethacin in hamsters. The gastroprotection by acidified Maalox against antral ulceration in this species corresponds well with the reported presence of its hexaaquoaluminum cation moiety at a pH below 4. Such gastroprotection may involve binding of this cation to the hamster pyloric antrum thereby protecting the antral mucosa against indomethacin-induced ulcerat