Cognitive training of auditory processing in schizophrenia induces significant improvement in cognition and plasticity in cortical activation patterns, but the biological mechanisms underlying these changes are unknown. In animals, intensive cognitive activity increases brain levels of the NMDA-receptor co-agonist D-serine, a molecular system which plays a key role in learning-induced neuroplasticity and which may be hypoactive in schizophrenia. We investigated whether training-induces cognitive gains were associated with increases in serum D-serine levels.