The interaction between free fatty acids and urapidil binding to human serum albumin (HSA) was investigated, whereby the binding properties at the coumarine binding site were influenced by the HSA fatty acid content. As has been reported for the coumarine derivative warfarin, the highest affinity of urapidil to HSA was found at the albumin fatty acid molar ratio of 1:3. Increasing the fatty acid concentration from 0 to 3 moles per mol human serum albumin caused an increase from 50% to 95% of bound urapidil. The affinity constant increased from 2.2 x 10(3) M-1 to 2.0 x 10(4) M-1 but the number of binding sites remained constant (n = 1.2). Therefore, one urapidil binding site per HSA molecule may be assumed.