D-serine is a physiological co-agonist of the N-methyl D-aspartate (NMDA) type of glutamate receptor, a key excitatory neurotransmitter receptor in the brain. D-Serine in the brain is synthesized from its L-isomer by serine racemase and is metabolized by the D-amino acid oxidase (DAO, DAAO), a flavoenzyme that catalyzes the oxidative degradation of D-amino acids including D-serine to the corresponding α-keto acids. The function of the NMDA receptor requires the presence of both the agonist (glutamate) and the co-agonist (D-serine, glycine, and/or D-alanine). Importantly, D-serine has been reported to be the predominant NMDA co-agonist in the forebrain and linked directly to schizophrenia. 1 D-serine concentrations in serum and cerebrospinal fluid have been reported to be decreased in schizophrenia patients, 2 and oral administration of D-serine improved symptoms of schizophrenia when used as an adjuvant to typical and atypical antipsychotics. 3 Thus, it is plausible to explore pharmaceutical inhibition of DAO function as putative novel therapeutics to treat the positive (psychotic), negative and cognitive symptoms in schizophrenia.