A method has been developed for the HPLC analysis of lipoic acid and its reduced form (dihydrolipoic acid) in biological samples. Both substances are released from the samples by enzymatic hydrolysis and extracted by solid phase column. The extracts, after evaporation, were chromatographed and quantified by electrochemical detection. The basic level was in the range 1-25 ng/ml for lipoic acid and 33-145 ng/ml for dihydrolipoic acid (6 healthy volunteers).