The purpose of this study was to determine whether alterations in the reactivity of gastric submucosal arterioles to norepinephrine and changes in acid secretion occur in the same time period in the rat. Experiments were performed on animals 6 weeks and 10, 18, and 25 months of age. Arteriolar diameter was measured by an image splitting in vivo microscopy technique before and after superfusion of the gastric submucosa with graded doses of norepinephrine. Acid secretion was measured in the pylorus-ligated rat. The percentage of arteriolar media stained with periodic acid-Schiff reagent (an age-related change) was determined for each age group. A dose-dependent response to norepinephrine was observed in 6-week and 10-, 18-, and 25-month age groups. Only in the 25-month-old rats were arterioles significantly more responsive to norepinephrine. Acid secretion at 10, 18, and 25 months was significantly less (about one-quarter) than that observed in the 6-week-old animals. Periodic acid-Schiff positive material deposited in the arteriolar media was significantly greater in the 18- and 25-month-old rats than in younger animals. We conclude that by 10 months of age in the rat, acid secretion is markedly reduced. The reactivity of the gastric submucosal arterioles is significantly increased at 25 months, but in the 18-month-old animal, when increased periodic acid-Schiff deposits are clearly evident, the reactivity to norepinephrine is not different from the 6-week-old rat.