In this study the influence of fasting on the structures in the human gastric mucosa was followed using selective histoenzymological and electronmicroscopic method. The gastrobioptical material of 7 healthy volunteers was examined after 24 to 240 hour fasting. During fasting an increased acid phosphatase activity in the chief cells of the human fundal mucosa was observed. The activity of the nonspecific esterase (naphtyl esterase) in the chief cells decreased mainly after 240 hour fasting. In the electronmicroscopic examination of the chief cells during fasting a multiplication of lysosomes and narrowing of the granular endoplasmic reticulum was observed. In some chief cells during fasting an agglomeration of zymogen granules was seen while in others only a few granules were observed. In the parietal and other cells of the human gastric glands a steatosis, which attained excessive values, was seen after 72 and 240 hours of fasting. We believe that this steatosis occurred mainly on the strength fasting lipemia with the possible participation of other factors. The changes in the mitochondrial structure of the parietal and other cells of the human gastric mucosa were only slightly pronounced. “The decreased” activity of dehydrogenase in the steatotic parietal cells under a histochemical examination may represent only findings caused by the agglomeration of a large amount of lipid drops in the steatotic cells. After 72 and 240 hours of fasting the parietal cells contained collapsed, underdeveloped intracellular canaliculi and narrowed tubulovesicular profiles probably related to the decreased HCl production. Generally it can be said that the observed morphological changes are the consequences of the decreased function of human gastric mucosa cells during fasting and that these regressive changes are reversible and they are caused by an insufficiency of nutrition.