A specific and sensitive procedure for determination of valproic acid at therapeutic concentrations in human plasma has been strandardized according to Hershey. The method requires solvents transfer but not solvent evaporation. For preparations of standard solution of high accuracy a sodium salt of valproic acid was used, titred potentiometrically with O.1 N percloric acid. Tests for sensibility and reproductibility were performed. The quality control of the assay was stablished using a control quality sera (Seronorm, Pharmaca).