An ulcer was induced in the anterior wall of the antrum by local injection of acetic acid solution. Carbonized microspheres 15 +/- 5 micrometer in diameter, labeled with 85Sr and 141Ce, were used to measure blood flow in different regions and layers of the stomach wall, and in each sample the mucosa was separated from the muscularis. The radioactivity of a blood reference sample and the tissue sample was determined, and the blood flow was calculated for each tissue sample. Two groups of anesthetized animals were used: animals with normal stomachs given vasopressin and animals with a 1-week ulcer given vasopressin. The vasopressin was administered intravenously over a 20-min period. In animals with normal stomachs and in animals with a gastric ulcer vasopressin was found to decrease the blood flow to the stomach in all areas examined. The presence of a 1-week ulcer in the cat did not seem to influence the effect of vasopressin.