The relationship between a reduced number of polymorphonuclear leukocytes (PMN), activated PMN/NBT (+) PMN/, permeability of gastric vessels, and disposition for the development of gastric lesions was studied following administration of indomethacin or ethanol in rats. Decrease in the number of circulating neutrophils and in the number of NBT (+) PMN after 3- or 6-day administration of methotrexate was found to diminish the disposition for the development of gastric lesions. Six-day administration of pentoxifylline resulted in a decreased number of NBT (+) PMN and in a diminished disposition for gastric mucosa injury induced by indomethacin, yet failed to reduce the occurrence rate of lesions after ethanol administration. No direct relationship was established between changes in gastric vascular permeability and decreased disposition for the development of gastric lesions after administration of methotrexate or pentoxifylline. (Fig. 3, Ref. 30.)