A sensitive and reliable method for the quantitative determination of itanoxone, 4-[4'-(2-chlorophenyl)phenyl]-4-oxo-2-methylenebutanoic acid, in biological fluids is described. A quantitative ethyl acetate extraction of the plasma samples is followed by reduction and methylation of itanoxone. Quantification is achieved by GLC using electron-capture detection and an internal standard. The minimum concentration of itanoxone detected in plasma is 0.1 microgram/ml. Recovery of the titrated compound added to human plasma averaged 100.9 +/- 2.92% (RSD).