15-Hydroxyprostaglandin dehydrogenase was measured in various gastrointestinal and non-gastrointestinal tissues from rabbits and rats. In addition, Δ13 reductase activity was measured in fundic and antral mucosae and gastric muscle from rabbits. In rabbits, antral mucosa contained the greatest activity of 15-hydroxyprostaglandin dehydrogenae and was 6 times more active than rabbit lung and fundic mucosa. In rat, duodenal mucosa was more active than antral mucosa or pancreas, suggesting that interspecies variations may exist. Δ13 reductase in rabbit gastric tissues was also more active in antral mucosa than in fundic mucosa or gastric muscle. The high activities of these enzymes in antral mucosa in conjunction with the known large prostaglandin content, particularly PGE2 and PGI2, suggest a large biological turnover of prostaglandin in these tissues. © 1981.