To get new information on the patho-physiology of the endocrine cells of the gastric mucosa and to investigate their role in the gastric secretion, an histochemical and electronmicroscopical investigation has been undertaken during experimental acute gastric hypersecretion induced in rats by pyloric ligation (Shay rats). Of the various types of endocrine cells, the ECL (entero-chromaffine-like) cells of the fundic mucosa showed ultrastructural evidences of marked activity (partial degranulation, formation of pro-granules, increase in ribosomes and enlargement of the rough endoplasmic reticulum). By histochemical techniques, cells interpreted as ECL cells showed a progressive decrease in argyrophilia, dopamine storage after L-DOPA administration, and histamine content. Other cell types, such as the A-like and EC cells were apparently unaffected. The antral G cells appeared, in the electron microscope, to be fully granulated and therefore probably inactive. It is concluded that, during hypersecretion in pylorus-ligated rats: Since the gastric hypersecretion in Shay rats is believed to be due to a vagal reflex, it can be suggested that the ECL cells, probably under a vagal control, play a role in the gastric secretion.