To evaluate the use of the fetal liver as an auxiliary graft, we have developed a model of intraabdominal heterotopic transplantation of late gestational fetal lamb livers into weanling lambs. Thirty-eight transplants have been performed of which 31 were technically successful. Twenty-three grafts functioned for intervals of 5 to 22 days after transplantation. Grafts were functionally evaluated by analysis of total bile acid and bilirubin excretion. To determine whether host liver excretory function would influence function of the graft, common bile duct ligated recipients were compared with recipients with normal host liver function. We found that (1) intraabdominal auxiliary transplantation of the fetal lamb liver is technically feasible; (2) the fetal liver graft is capable of rapid adaptation and can assume a significant portion of host excretory function; and (3) excretory function of the fetal liver is proportional to the functional demands of the host. Auxiliary transplantation of the fetal liver is a promising alternative to current methods of liver transplantation.