Glycine is an agonist at brain N-methyl-D-aspartate receptors and crosses the blood–brain barrier following high-dose oral administration. In a previous study, significant improvements in negative and cognitive symptoms were observed in a group of 21 schizophrenic patients receiving high-dose glycine in addition to antipsychotic treatment. This study evaluated the degree to which symptom improvements might be related to alterations in antipsychotic drug levels in an additional group of 12 subjects. Glycine treatment was associated with an 8-fold increase in serum glycine levels, similar to that observed previously. A significant 34% reduction in negative symptoms was observed during glycine treatment. Serum antipsychotic levels were not significantly altered. Significant clinical effects were observed despite the fact that the majority of subjects were receiving atypical antipsychotics (clozapine or olanzapine). As in earlier studies, improvement persisted following glycine discontinuation.