Serine racemase (SR) generates d -serine, a coagonist with glutamate at NMDA receptors. We show that SR is physiologically S -nitrosylated leading to marked inhibition of enzyme activity. Inhibition involves interactions with the cofactor ATP reflecting juxtaposition of the ATP-binding site and cysteine-113 (C113), the site for physiological S -nitrosylation. NMDA receptor physiologically enhances SR S -nitrosylation by activating neuronal nitric-oxide synthase (nNOS). These findings support a model whereby postsynaptic stimulation of nitric-oxide (NO) formation feeds back to presynaptic cells to S -nitrosylate SR and decrease d -serine availability to postsynaptic NMDA receptors.