Binding equilibria of valproate (2-n-propyl-pentanoic acid anion) with defatted human serum albumin were studied by equilibrium dialysis in a 66 mM sodium phosphate buffer, pH 7.4, 37 degrees. Three hundred and fifty-six observed points for bound versus free valproate concentration were obtained and analyzed in terms of stepwise binding. It was found that the best fit resulted from a model in which 67% of the albumin was capable of binding valproate, whereas 33% did not bind. Thirty acceptable variants of the curve fitting were generated in order to assess the variation of the binding constants. The binding albumin component combines with three molecules of valproate with high affinity and with at least seven additional molecules that are loosely bound. Saturation of the protein cannot be reached. At very high concentrations of free valproate (above 10 mM) irreversible changes in the albumin take place, resulting in poor reproducibility in the amount of bound valproate. In the presence of palmitate, 0.5, 1, and 1.5 mol/mol of albumin, binding of valproate is decreased by a competitive mechanism. It is hypothesized that obesity, developing as a complication of valproate treatment of epilepsy, results from increased availability of long-chain fatty acids due to competitive valproate binding.