GS4071 is a potent inhibitor of influenza neuraminidase. A precolumn fluorescence derivatization HPLC method is described for the analysis of GS4071 in rat plasma. Plasma samples were subjected to solid-phase extraction on C18 extraction columns. After extraction, GS4071 was derivatized with naphthalenedialdehyde in the presence of potassium cyanide to produce highly fluorescent cyano[f]benzoisoindole derivatives. Derivatized samples were stable for >24 h at 4 degrees C. The samples were analyzed by an isocratic HPLC method using fluorescence detection at 420 nm excitation and 470 nm emission wavelength. The method was validated and applied to the analysis of plasma samples from pre-clinical pharmacokinetic studies in rats. The limit of detection for GS4071 was 20 ng/ml. For five replicate samples at 50, 400, and 1000 ng/ml, the within-day precision values were 16.9, 9.4 and 4.5%, respectively, and the between-day precision values were 16.9, 7.9, and 2.1%, respectively. The method was linear from 25 to 1600 ng/ml and the total recovery was >68% over this concentration range.