Cell proliferation of gastric antral and fundic mucosa was studied in a series of patients with different gastric diseases. After in vitro incubation of endoscopic biopsies with tritiated thymidine, radioautographs were prepared by the dipping technique. The labeling indices were estimated, and the biopsies were classified according to the degree of gastritis. The peak acid output after "maximal" stimulation with histamine was also determined. A highly significant inverse correlation between peak acid output and the labeling indices in both antral and fundic mucosa was demonstrated. The correlation between the degree of gastritis and the labeling indices in antral and fundic mucosa was also statistically significant. The results indicate that a significant inverse relationship exists between the acid secretory capacity of the gastric mucosa and the rate of cell proliferation in both antral and fundic mucosa. Furthermore, the data suggest that an increasing degree of gastritis in human gastric mucosa is followed by an increased rate of epithelial cell proliferation.