1. Seven normal volunteers took 0.28–0.42 mmol (100–150 mg) of deoxycholate by mouth. This resulted in a reduced proportion of chenodeoxycholate in bile and an increased proportion of deoxycholate. Cholate was unchanged. 2. Cholate and chenodeoxycholate pools and rates of synthesis were determined in four of the subjects by simultaneously labelling each pool with relabelled bile acids. The chenodeoxycholate pool and rate of synthesis decreased after deoxycholate administration. Cholate synthesis and pool size did not change appreciably.
3. The proportion of deoxycholate in bile samples of sixty-two subjects with intact enterohepatic circulation was found to be inversely related to the proportion of chenodeoxycholate in bile, but not to the cholate.
4. It is suggested that inhibition of chenodeoxycholate synthesis by deoxycholate, the principal bacterial product of cholate, regulates the size of the chenodeoxycholate pool independently of the total amount of bile salt.