Quantitative morphological data on normal human oxyntic mucosa were obtained from endoscopic biopsies in ten healthy male volunteers. Corpus mucosa was biopsied in the resting state and during maximal acid secretion and then processed for light and electron microscopy. Stereological analyses were carried out on sections comprising the entire thickness of the epithelial layer. About one-third of the mucosal volume was taken up by lamina propria and 15% by parietal cells. Counts of cells that displayed their nucleus in the sections revealed that in average of 12% of the epithelial cells were parietal cells, 43% were mucous cells, 40% were zymogen cells, and 4% were endocrine cells. Parietal cells displaying two nuclei were twice as large as those with only one nucleus. Six percent of the parietal cell volume was taken up by the nucleus, and 33% of the cytoplasmic volume was occupied by mitochondria. Stimulation of acid secretion resulted in a 76% increase in the secretory surface density; simultaneously there was a slight decrease in the mean size of the parietal cells and an increase in the relative volume of the nucleus. During maximal stimulation of acid the parietal cells from the superficial mucosal layers displayed a 40% larger secretory surface than those from the deeper parts of the mucosa. The data, which will serve as a basis for studies of pathological mucosae, are compared with those obtained in other species.