Bile acids in the serum of rat portal blood have been examined. Fasting (20 hr) caused a marked increase in the deoxycholic acid concentration, mainly of deoxycholyltaurine. Studies with [24-14C]deoxycholic acid failed to show that this was due to decreased rehydroxylation by the liver. Resection of the terminal ileum caused a three- to fourfold reduction in the total bile acid concentration in male animals, cholyltaurine being most affected although it remained the predominant bile acid. In contrast the concentration of deoxycholic acid increased in half the animals. In female animals the concentrations of chenic acid and lithocholic acid also increased so that the total bile acid concentration only decreased slightly. Ileal resection combined with a right hemicolectomy caused the disappearance of deoxycholic acid. We would like to thank Mrs. J. Paxton and Mr. E. Shaw for assistance in the animal studies, and Miss J. Jackson for technical assistance. Professor S.N. Huang kindly reviewed the histological liver specimens. © 1976, SAGE Publications. All rights reserved.