Administration of cyclosporin A (CsA) may induce cholestasis, and this effect has been attributed to impaired hepatocellular uptake, transport, secretion and intestinal absorption of bile acids. Disturbances of the enterohepatic circulation may affect metabolism of bile acids. To test whether liver transplantation and treatment with CsA alters pool sizes or synthesis and turnover rates, we determined kinetics of primary bile acids in patients after orthotopic liver transplantation on CsA. Two male and four female patients were studied 6-20 months after transplantation. They had no overt signs of cholestasis, graft dysfunction or rejection. Kinetics of cholic acid (CA) and chenodeoxycholic acid (CDCA) were simultaneously determined after oral administration of [24-13C]-CA and [24-13C]-CDCA on the basis of isotope dilution in a single pool of bile acids. Ten healthy volunteers served as controls. After orthotopic liver transplantation, pool sizes, fractional turnover rates and synthesis rates of both primary bile acids, CA and CDCA were not significantly different from control subjects. In spite of the known interference of CsA with the enterohepatic circulation of bile acids, in the majority of patients after orthotopic liver transplantation without cholestasis, graft dysfunction of rejection, treatment with CsA does not disturb kinetics of primary bile acids.