The protein-binding characteristics of the angiotensin II receptor antagonist valsartan were determined in vitro by equilibrium dialysis, using 14C-labeled valsartan with serum from healthy donors, plasma from patients who had received valsartan, serum or plasma from animals, and purified human plasma proteins. The binding of valsartan was high (96 +/- 2%) in human serum at concentrations ranging from 0.05 micrograms/mL to 5 micrograms/mL. A comparable extent of binding (85-99%) was recorded in plasma from patients after repeated administration of valsartan. Albumin (binding 92%) is the main protein involved in the binding to plasma proteins, while the binding to alpha 1-acid glycoprotein was low (22%) and to gamma globulins, negligible. Although highly bound, valsartan was not displaced in vitro by hydrochlorothiazide, diclofenac, furosemide, and warfarin. A high extent of binding was found in rat, dog, and rabbit serum and in marmoset plasma, while a lower binding was found in mouse serum.