1. 1.Measurements were made of the urease (urea amidohydrolase, EC 3.5.1.5) content of the gastric mucosae of conventional, germ free, and fetal animals to test whether gastric urease arises from micro-organisms or is constitutive. 2. 2.Homogenates of gastric mucosae of conventional sheep, dogs, cats, rats, guinea pigs, and chickens; of germ-free rats, guinea pigs, chickens, dogs, cats, and pigs; and of fetal dogs, cats, rats, sheep, and humans were prepared and analyzed for urease activity. 3. 3.There was no urease in the gastric mucosae of the germ-free or fetal animals, although enzymic activity was usually found in the same tissue from their conventional adult counterparts, which had a wide range of enzymatic activity. © 1968.