A highly sensitive method for the measurement of malondialdehyde as thiobarbituric acid-malondialdehyde complex by reversed-phase high-performance liquid chromatography with fluorescence detection in biological samples is described. As samples, 20 microliters of rat plasma or 10% (w/v) liver homogenate mixed with 2.0% thiobarbituric acid in 2 M sodium acetate buffer containing 0.05% butyl hydroxytoluene (pH 3.5) were heated at 95 degrees C for 45 min to give the complex. The complex, extracted with n-butanol, was chromatographed on a system equipped with a reversed-phase column, and the eluted peak was monitored with a fluorescence detector (excitation wavelength 515 nm, emission wavelength 553 nm). The mobile phase was a acetonitrile-water (2:8, v/v) under isocratic conditions at ambient temperature, and a single analysis was done in ca. 4 min. The minimum detection level for malondialdehyde was as low as 0.05 pmol. The n-butanol extract was stable at least for 3 days. The simple mobile phase, the extremely sensitive detection limit, and the stability of the complex make this system applicable to routine clinical analysis with a small amount of tissue or biopsy sample.