The binding parameters of valproic acid (VPA) to human serum albumin (HSA) were determined by equilibrium dialysis and computed using non-linear regression. However unclassic, it was observed that the binding parameters of VPA varied according to the concentration of the HSA solutions used. At 580 microM HSA, VPA is bound to two classes of binding sites with the association constants K1 = 56000 M(-1) and K2 = 750 M(-1), and their respective numbers of binding sites n1 = 2 and n2 = 5. Free serum fraction of VPA is significantly increased by 500, 1000 and 1500 microM palmitate, 250 microM clofibrate, 320 microM phenylbutazone, or 360 microM salicylate. In any case, the increase of the VPA serum free fraction is highly correlated with the inhibitor concentration. On the other hand, the free serum fraction of warfarin is increased by 600 microM VPA (100 micrograms/ml). In patients with liver disease, the variations of the free serum fraction of VPA are correlated to the albumin and to the bilirubin concentrations. Serum binding of VPA is significantly decreased in the two groups of patients as compared with the control group.