A method for the determination of D- and L-tryptophan (Trp) in biological samples is described. The amino acid enantiomers were precolumn-derivatized with a fluorescence tagging reagent, naphthalene-2,3-dialdehyde (NDA). In the presence of hydroxypropyl-gamma -cyclodextrin (HP-gamma -CD) as the chiral selector, NDA-tagged Trp enantiomers were well resolved by micellar electrokinetic chromatography (MEKC). Using laser induced fluorescence (LIF) detection, a detection limit of 3.3 x 10(-8) M Trp was obtained. The method was applied to the determination of Trp enantiomers in biological samples including human urine and cerebrospinal fluid (CSF), rat brain tissue, and Aplysia ganglia. No interference from other amino acids or the endogenous compounds in the sample matrices was observed. D-Trp was found at the sub-pm level in human urine samples collected from several healthy subjects. Further, the determination of DL-Trp residues in small quantities (10 mug) of peptides after acid hydrolysis is demonstrated.