Background: The severity of mucosal damage after horizontal gastroplasty is closely related to the high PH of the pouch. Cimetidine proved to promote healing of acute gastric erosions. Aim of the Work: This research was carried out to investigate the effect of horizontal gastroplasty on the stomach mucosa and protective effect of cimetidine. Materials and Methods: A total of 15 male adult rats were divided into three equal groups. The first one considered as control. The second group was exposed to horizontal gastroplasty. The third group was exposed to horizontal gastroplasty with immediate injection of Cimetidine in a dose of 200 mg/kg/ daily for one month. The rats were sacrificed after the last dose of the drug and the stomach was removed and different sections were prepared for light and electron microscopic investigation. Results: Examination of the gastric mucosa in the control group revealed the traditional mucosal architecture. In the second group using L/M examination, the different types of cells of gastric glands were distorted with condensation of their nuclei and disruption of the epithelium of gastric glands. There was a focal area of inflammation with marked increase of inflammatory cells and areas of congestion. The lumens of gastric glands were markedly dilated and the connective tissue between the glands was increased. By E/M examination, some nuclei appeared small and condensed .The cell membrane surrounded the cells disappeared, the Golgi apparatus was dilated, the mitochondria were swelled with distortion of their cristae and the rER were distended with shedding of their ribosomes. The microvilli disappeared and secretory granules were scanty with small clear and irregular vacuoles. In the third group, L/M and E/M examinations revealed that most affections appeared in the second group have vanished.