Bisphosphonates are a family of chemically related zwitterionic molecules that are used clinically to retard bone resorption in individuals with osteoporosis and associated skeletal diseases. Inflammation and ulceration of the upper gastrointestinal tract by a mechanism that relates to a topical irritant action is associated with the consumption of some bisphosphonates. In the present study, we investigated the effects of three bisphosphonate molecules, pamidronate, alendronate, and risedronate on the surface hydrophobicity and phosphatidylcholine (PC) concentration of the antral mucosa. We also examined how these surface changes related to mucosal injury in an established rat model, in which the test compounds were administered in combination with indomethacin. We initially determined that a combination of pamidronate (300 mg/kg) and indomethacin (40 mg/kg) induced a significant reduction in mucosal surface hydrophobicity and macroscopic lesion formation by 15 min and mucosal PC concentration by 30 min, with the magnitude of these changes increasing over the 4-hr study period. An equivalent dose of alendronate or risedronate in combination with indomethacin produced modest or no macroscopic injury, respectively, to the antral mucosa over the 4-hr study, although the bisphosphonates clearly induced surface injury and some glandular necrosis when examined at the light microscopic level. These bisphosphonates also induced modest decreases in antral surface hydrophobicity and mucosal PC concentration that appeared to be related to their injurious potential. In conclusion, the variable toxicity of bisphosphonates to the antral mucosa appears to be associated with their ability to compromise the surface hydrophobic phospholipid barrier of the tissue, with pamidronate > > > alendronate > risedronate. This bisphosphonate effect on the surface barrier may trigger the development of mucosal injury and possible ulceration.