Following deproteinization of plasma with organic solvents the supernatant was shaken with hexane and cation-exchange resin in an Eppendorf tube to remove fatty and amino acids and the medium was subjected to direct treatment with ethyl chloroformate under catalytic influence of pyridine. A subsequent extraction of the immediately formed ethyl esters with a drop of chloroform enabled us to subject the sample to gas chromatographic (GC) analysis. Since ketocarboxylic acids do not require a preliminary oximation the total time of sample workup and analysis takes only several minutes.