An isolated in vivo rat cecal loop technique was utilized to determine what structure of bile acids is required to stimulate net colonic secretion of water and sodium. A dose response curve for water and sodium movement was determined for deoxycholic acid (1-6 mM) and chenodeoxycholic acid (3-6 mM). Both of these bile acids were associated with significant secretion of water and sodium at 4 mM concentration. Therefore, this concentration was used for all test bile acids studied. Test solutions included the tri- and di-substituted bile acids: cholic acid, hyocholic acid, 3 alpha, 7 beta, 12 alpha-trihydroxycholanic acid, 3 alpha, 12 alpha-dihydroxy-7-ketocholanic acid, ursodeoxycholic acid, 3 alpha-hydroxy-7-ketocholanic acid, 7 alpha, 12 alpha-dihydroxycholanic acid, and hyodeoxycholic acid. Only three bile acids, deoxycholic acid, chenodeoxycholic acid, and 7 alpha, 12 alpha-dihydroxycholanic acid were associated with net secretion of water and sodium. Cecal histology after incubation with bile acids revealed mucosal alterations in those sacs in which secretion occurred, but not where absorption was noted. These data indicate that bile acid-associated water and sodium secretion in the rat cecum requires a specific bile acid structure with a definite spatial relationship of the hydroxyl groups. Secretion occurred only with two alpha-hydroxyl groups in either the 3, 7, or 12 positions.