Abstract: Plasma was mixed with 3,3-difluoro-D-alanine(I) as internal standard, and transferred to an ultra-filter and centrifuged; the filtrate was then analyzed. Urine plus I was mixed with acetonitrile and applied to a Sep-Pak silica column and the fraction eluted with water was analyzed. Analysis was by HPLC on a Radial-Pak C18 cartridge (10 cm x 8 mm; 5 µm) with a Guard-Pak C18 cartridge and a mobile phase (2 mL min-1) of Na dodecyl sulfate (50 mg mL-1) in aqueous 10% methanol containing 2 mL L-1 of 85% H3PO4, at pH 2.5. Post-column derivatization was achieved with phthalaldehyde and 2-mercaptoethanol in borate buffer solution in a glass-bead-packed reactor at 40°C with fluorimetric detection at 455 nm (excitation at 340 nm). The calibration graphs were rectilinear for 0.25 to 20 µg mL-1 (plasma) and 0.5 to 200 µg mL-1 ( urine). The intra- and inter-assay coefficient of variation were 6.14 and 8.7%, and 5.18 and 8.3%, for plasma and urine, respectively; the respective recoveries were 48.9 and 66.5%.