A new method for the detection of free and total malonaldehyde (MDA) in human plasma samples based on the derivatization of MDA with 9-fluorenylmethoxycarbonyl hydrazine (FMOC-hydrazine) in an acidic medium was developed. Derivatization was achieved after 4 h at 50 degrees C. The derivatized samples were analyzed by HPLC using a reversed-phase C18 column with fluorescence detection (Ex=270 nm, Em=310 nm). The benefit of this direct injection of deproteinized plasma is to avoid the use of an internal standard. The detection limit was 0.1 pmol (4.0 nmol/L). The recovery of MDA spiked in different human plasma samples was 95.3% (n=25; R.S.D. 5.1%) for the hydrolysation procedure. The total and free MDA in plasma of 15 healthy male volunteers are 426+/-29.8 nmol/L and 153+/-9.6 nmol/L, respectively.