We detected platelet‐activating substance in gastrointestinal areas, which was confirmed to be platelet‐activating factor (PAF) on the basis of the following findings:1) it comigrated with authentic PAF on thin‐layer chromatography;2) it did not aggregate PAF‐desensitized platelets; and3) its activity was completely antagonized by the receptor antagonists CV3988 and L‐652,731. The level of PAF was determined with a bioassay method based on the release of [3H] serotonin from washed rabbit platelets. In the normal rat stomach, the level of PAF was high in the antrum (940 ± 200 nmol PAF/mol phosphorus of original phospholipids), especially in the antral mucosa (1801 ± 426 nmol/mol phosphorus of original phospholipids). The stomach PAF level was significantly altered by water immersion stress. Stress for a period of 1 h was associated with a decrease in the antral PAF level to 39 ± 7% of that of untreated controls. This low PAF level persisted during stress. On the other hand, in the corpus, stress for periods of 1 and 3 h was associated with decreases in the PAF content, and further stress (7 h) resulted in restoration of the PAF level to normal. Furthermore, 7 h of stress was associated with distinct hemorrhagic lesions, which were prevented by CV3988 infused i.v. before the stress. This is the first report of an association between a decrease of the endogenous PAF level in animal tissues and tissue damage.—Sugatani, J.; Fujimura, K.; Miwa, M.; Mizuno, T.; Sameshima, Y.; Saito, K. Occurrence of platelet‐activating factor (PAF) in normal rat stomach and alteration of PAF level by water immersion stress.FASEB J.3: 65‐70; 1989.