The effects of dry air, saline, and ileostomy fluid containing trypsin were independently measured on two types of ileostomies in 16 dogs for two weeks. In all instances, the everting type of ileostomy retained its original length, pliability, and patency. The ileostomies with exposed serosa underwent up to 92% shortening with stenosis and rigidity when exposed to fluid containing pancreatic enzymes. The proteolytic activity of ileostomy fluid was found to liberate from 0.128 to 0.396 millimol of tyrosine per liter per minute with the Anson-Mirsky method. Air exposure or constant saline moisture resulted in lesser shortenings, 37% and 40% respectively, in ileostomies with serosa exposed.