Serine racemase (SR) is an enzyme responsible for the biosynthesis Of D-serine, the coagonist of the N-methyl-D-aspartate receptor, in the brain. Therefore, it has been suggested as a possible therapeutic target for the treatment of various neurodegenerative diseases. To develop a potent inhibitor of SR, a simple, sensitive, fast, and robust assay is needed. In this paper, a new CE method for the determination of D-serine is described. Serine enantiomers are resolved in the form of o-phthaldialde-hyde (OPA)/2-mercaptoethanol (2-ME) derivatives in an alkaline BGE composed of 50 mM sodium tetraborate, pH 9.7, and containing 40 mM 2-hydroxypropyl-gamma-CD as a chiral selector. The problem of time-limited stability of OPA/2-ME derivatives has been overcome by employing in-capillary derivatization of the sample, i.e., the derivatization reaction was carried out directly in the separation capillary in the first phase of the CE run. UV-absorption detection at 230 nm allowed concentration detection limit of 3 mu M. Baseline resolution Of D- and L-serine derivatives was achieved in less than 10 min. This fact, together with the simple sample pretreatment, allowed application of the method to medium-throughput monitoring of SIR activity, such as the screening of potential SR inhibitors. A good agreement was achieved between the developed CE method and the previously established HPLC method for determination of the inhibition constant, K-i, of a new SIR inhibitor, L-erythro-3-hydroxyaspartate.