The protective effects of exogenous phospholipid on aspirin-induced gastric mucosal injury were examined in a canine chamber model which provided two separate segments of mucosa supplied by a single vascular pedicle. In each dog, one segment was treated with a suspension of surface-active phospholipid, similar in composition to that normally present in the gastric mucosa, whereas the other segment served as the control. Pretreatment of the test segments significantly prevented aspirin-induced disruption of the mucosal barrier as evidenced by an increase in potential difference and a decrease in acid back-diffusion and sodium ion and potassium ion flux. These findings were associated with a marked reduction in the degree of mucosal injury. Our results support the recent hypothesis that surface-active phospholipid plays an important role in gastric mucosal defense against the damaging effects to luminal acid.