The secretory activity of surface mucous cells was quantitatively studied in the mouse stomach under three different dietary conditions: ad libitum feeding, fasting for 15 hrs, and refeeding 1 hr after 15-hrs fast. Surface mucous cells were classified into isthmus cells, foveolar cells, surface cells and degenerating cells according to stage of maturation. The number of exocytosis and cytoplasmic granules was calculated in unit length of the apical plasmalemma for cells in each stage. Foveolar and surface cells in fasted animals manifested higher exocytotic activity than the other two groups (P less than 0.01). This suggests that physical and chemical stimuli of the gastric content may greatly affect the secretory activity of the cell. Although the number of cytoplasmic mucous granules proved largest in the upper part of the foveola and less at the mucosal surface under any dietary condition, exocytotic activity did not differ significantly between the foveolar cells and surface cells. Degenerating cells very actively discharged mucus, regardless of dietary condition. It is reasonable to postulate that the cells secrete mucus in order to cover the surface and protect the mucosa from damage during degeneration.