The Peneth cells in intestinal metaplasia of stomach and the duodenum in human subjects were studied ultrastructurally, and the fine structures of these cells were compared. Paneth cells showed the ultrastructure of serozymogenic cells and secreted their secretory granules by merocrine process. The rod or tubular dense bodies were observed in the apical region of some Paneth cells. The structures may have some relation to the secretion of the secretory granules. The secretory granules with less dense layer in the periphery, which had never been described in the Paneth cell of man, were also observed. Morphologically intermediate cells between Paneth cell and goblet cell were found. Some of the Paneth cells might be phagocytized by undifferentiated crypt cells. The Paneth cells in intestinal metaplasia were fundamentally the same as those in duodenum at least in morphology. Difference between them was that Paneth cells with many phagolysosomes in the lower cytoplasm were observed more frequently in the duodenum than in intestinal metaplasia of the stomach. The physiological functions of the Paneth cell have been discussed.