An in vivo intestinal perfusion system was used to study the effects of different bile acids on fluid secretion, mucosal permeability, and mucosal morphology in the rabbit colon. To define the structure-activity relationships of the bile acids, nine unconjugated bile acids were used, varying only in the number (two or three) or position (3, 7, or 12 or various combinations) of hydroxy or keto nuclear substituents. Results showed that bile acids with two hydroxy groups in the alpha configuration at the 3,7 position, 3,12 position, or 7,12 position induced fluid secretion, increased mucosal permeability, and produced mucosal damage as assessed by light and scanning electron microscopy and quantitated by DNA loss during perfusion. Replacement of hydroxy groups by keto groups or a change from alpha to beta configuration for the hydroxylic substituent in the 7 position abolished all three activities. Trisubstituted derivatives, whether hydroxy or keto, did not affect fluid secretion permeability or cause mucosal damage. These studies indicate that of the major primary and secondary bile acids in man, only deoxycholic and chenodeoxycholic acids alter colonic structure and function in the rabbit. They show further that the cathartic effects of bile acids have specific structural requirements; and they show that bile acid-induced secretion was invariably associated with increased mucosal permeability and epitheliolysis.