In this study, an attempt was made to localize the cell types and subcellular organelles that synthesize and secrete surface-active lipids from the rat gastroduodenal mucosa. Two fluorescent hydrophobic probes--Nile Red and 1-anilinonaphthalene-8-sulfonic acid--and a phospholipid-selective cytochemical stain--iodoplatinate--were used for fluorescence and electron microscopic studies, respectively. The results showed the presence of phospholipid and neutral lipid fluorescent hot spots in the mucous cells of both the gastric mucosa and the submucosal Brunner's glands of the duodenum. In contrast, other cell types of the stomach (parietal, chief, and endocrine cells) or the duodenum (goblet cells and villous and crypt enterocytes) were either unreactive or weakly stained with the dyes. Ultrastructurally, two classes of large infranuclear inclusion bodies were observed in these two types of mucous cells. The location and size of these inclusion bodies appeared to be in agreement with the position and size of the large hydrophobic fluorescent hot spots detected at the light microscopic level. The ultrastructural appearance, localization, and histochemical staining pattern of these lipid-containing organelles suggest that they are specific for these cell types and not a general feature of degenerative epithelium undergoing lysosome-induced autolysis. It was concluded that the mucous cells of both the stomach and the Brunner's glands contain a subcellular organelle that stores hydrophobic material constituting both neutral lipids and phospholipids. These lipidic substances may be secreted into the gastroduodenal mucous gel and provide its surface with a nonwettable lining to repel luminal acid.