The chromone FPL 52694 is a preparation thought to act by stabilizing mast cell membranes, thus reducing free histamine in the gastric mucosa. The effect of FPL 52694 on overnight gastric secretion was tested in 15 dyspeptic individuals in a double-blind crossover study. Each individual received the chromone preparation or placebo for 1 week. After each of the treatment periods gastric secretion was collected after 6 h of fasting by continuous suction for 8 h during the night. A significant reduction of the total acid secretion was observed after active drug compared with placebo administration, mean reduction being 17% (median, 23%). No difference was observed between 30-min and 60-min periods. FPL 52694 had no significant effect on pepsin, intrinsic factor (IF) or volume secretion. The difference in the effect on pepsin and acid secretion, reflected in a significant increase in the pepsin to acid ratio, indicates that intrinsic histamine, similarly to extrinsic histamine, has a specific effect on parietal cells. The difference in effect on acid and IF secretion indicates that intrinsic histamine not only has a specific effect on parietal cells but also has different effects on the secretory substances contained in this cell. Since IF was not reduced by the use of FPL 52694, it may be that this substance will not influence vitamin B12 metabolism.