The influence of bile salts on the mucosal surface of rat jejunum was tested with an in vivo technique of segmental perfusion. Sodium taurocholate and chenodesoxycholate were applied in a concentration of 3 mmol/l. The release of 5 brush border membrane enzymes, 5 cytosolic, 1 mitochondrial, and 2 lysosomal enzymes during a perfusion time of 150 min as well as morphological alterations after bile salt treatment were investigated. Among the membrane enzymes, due to their superficial localization, the solubilization of enteropeptidase and alpha-1,4-glucosidase was highest both in the control perfusion and in the presence of bile salts. At the same time, cytoplasmic enzyme activities were liberated extensively whereas lysosomal and mitochondrial enzymes were scarcely detectable. This disproves any serious injury of the enterocytes. Electronmicroscopic results supported this suggestion. After administration of taurocholate (in physiological concentration), only an occasional diminution of the glycocalyx was observed and even chenodeoxycholate (in an unphysiological concentration) caused only negligible destructions of intestinal brush borders. Investigations with ruthenium red to contrast the glycocalyx showed a partially unchanged structure. Microvesiculation from the microvilli was observed in many electron microscopic photographs. That is a possibility for the release of membrane-bound and cytosolic enzymes without destruction of enterocytes.