The experimental caustic effects on the rat's esophagus were produced with 1.0 cc of 2.0% sodium hydroxide solution and 1.0 cc of 3.0% acetic acid solution. The histopathological findings and tissue mast cells of esophagus and stomach have been surveyed, and the influence of administration of hydrocortison were observed. 1) The number of tissue mast cells appeared on the surface of a round cut of esophageal cardia were 65.4±2.87, and they consisted of 96.1% of normal form of mast cells and 1.2% of moderately diffused ones. 2) The average value of the 20 microscopic fields of the mast cells appeared on the surface of a round cut covering from the middle part of the small curvature to the large curvature in stomach were 31.0±2.80 and they consisted of 96.1% of normal ones, 3.9% of moderately diffused ones, and none of highly diffused ones. 3) The administration of sodium hydroxide and acetic acid solution made the number of the mast cells decreased, accompanying with the severe histopathological changes from the first day, to the minimum on the third day, when there developed a marked inflammation. Since then it became to increase gradually beyond that of the controlled group at the end of second week, and finally returned to that of the controlled group by the end of fifth week. 4) In case of corrosive esophagitis due to sodium hydroxide demonstrated more severe histopathological changes as well as marked difference in the number of mast cells on esophagus than stomach. On the contrary, in the case of acetic acid, stomach showed the marked changes more than esophagus. 5) For the ratio of appearance of each type of mast cells, the normal form of them were demonstrated generally in the lowest rate on the third day-group, accompanying with the severe inflammatory findings. On the contrary, the diffused ones were found in the highest rate, however, in the case of esophagus of sodium hydroxide, the recovery was delayed comparatively. 6) In case of esophagus of sodium hydroxide, demonstrated the lower appaerance rate of normal ones and the higher rate of the diffused ones, but in case of stomach the adminstration of acetic acid showed the lower rate of normal ones and the higher rate of diffused ones. 7) The administration of cortison showed the lighter of histopathological changes and the minimized difference in the number of mast cells and made them appear in lesser degree. The rising of the appearance ratio of the diffused ones have been lessened with the administration of the cortison. Hereby we can easily recognize that the cortison has some role in controlling the inflammation as well as the abnormal appearance of mast cells.