Rat intestinal mucin was labelled biologically by intraperitoneal injection of radioactive amino acids and monosaccharides 3--6 h prior to killing, followed by isolation and purification of the mucin from mucosal scrapings. The labelled product was then introduced into intestinal segments of rats under ether anesthesia for periods up to 3 h, removed by washing and assessed for evidence of degradation. In segments containing the pancreatic ducts the total mucin precipitable by cetyltrimethylammonium bromide fell from 80% to 5% in 3 h. At 3 h, chromatography on Sephadex G-100 and Sepharose 4B revealed multiple products, including very small molecular weight fragments deficient in carbohydrate label. With the introduction of neomycin sulfate into the segments to reduce bacterial growth, only two products were found, one corresponding in size to the original mucin and one somewhat smaller, although still in excess of 200,000 daltons. These products occurred independently of the presence of the pancreatic ducts in the segments, and in chronically pancreatectomized rats. The smaller product could not be produced by incubation with trypsin or elastase. Both products were altered antigenically as compared with the original mucin. Both products also retained the same ratio of carbohydrate and protein label as the original. It is concluded that mucins undergo early degradative changes in the intestine which do not involve deglycosylation but which involve partial loss of antigenicity and a fall in molecular weight. The pancreas is not responsible for these changes.