Serotonin in doses of 5 to 80 mg/kg injected subcutaneously to rats injured the gastric mucosa dose-dependently and the lesion was histopathologically regarded as erosion. Aggregated platelets were observed in small blood vessels of the gastric mucosa prior to the development of erosion when serotonin was injected in a subcutaneous dose of 20 mg/kg effective in producing the lesion. Subsequently fibrinous thrombi could be found in the lesional mucosal area at the stage of erosive change. Serotonin in the same dose decreased gastric mucosal blood flow and enhanced gastric motility in the peroid of time when pre-erosive or erosive change occurred. Circulatory disturbance of the gastric mucosa induced by these histological and functional changes seemed to play an important role in development of the gastric mucosal lesion by serotonin. It is unlikely that gastric acid takes part as a trigger in producing the lesion.