 Several lines of evidence implicate glutamatergic dysfunction in the pathophysiology of obsessive compulsive disorder OCD. Sarcosine, also known as N-methylglycine, is an endogenous antagonist of glycine transporter-I GlyT-I, which potentiates glycines action at the glycine site of N-methyl-D-aspartate NMDA receptors. In this 10-week open-label trial, we examined the efficacy and safety of sarcosine treatment in OCD patients. 