We describe a method for the detection and quantification of D-aspartate N-methyltransferase activity. The enzyme catalyzes the S-adenosyl-L-methionine-dependent N-methylation of D-aspartate to form N-methyl-D-aspartate (NMDA). NMDA is detected directly by high-performance liquid chromatography (HPLC) of their (+)- and/or (-)-1-(9-fluorenyl)ethyl chloroformate fluorescent derivatives. The NMDA production in the assay mixture is linearly proportional to the incubation time and the amount of tissue homogenate. Using a 10 min incubation time, the method allows detection of the enzyme activity below 10 fmol/min. It can be used to analyze kinetic behavior and to quantify the enzyme from a wide variety of organisms. (C) 2011 Elsevier B.V. All rights reserved.