The proposed experiment will evaluate the effects of the NMDA receptor full agonist D-serine (~2g/day) on persistent symptoms of Parkinson's Disease and on antiparkinsonian drugs-induced dyskinesias.

D-serine will be used as add-on therapy to on-going medications received by Parkinson's Disease patients. The rational for this study stems from observations made in pervious clinical trials with schizophrenia patients, in which it was demonstrated that D-serine adjuvant treatment resulted in:1)improvement of parkinsonian side effects induced by antipsychotic drugs and 2) improvement of depression and negative (i.e apathy, blunted effects, anhedonia) symptoms which are similar to symptoms encountered in Parkinson's Disease.

The study will have a crossover design in accordance to which each patient will receive, in random order D-serine and placebo for a 6 weeks period each. Thus, any participant will have the opportunity to receive the experimental treatment.

Inclusion Criteria:

• PD diagnosis
• ≥2 on UPDRS items 32,33
• receive treatment with L-dopa alone or in combination with other antiparkinsonian medications.

Exclusion Criteria:

• current or previous history of other neurological disorders
• unstable medical conditions
• renal pathology
• pregnant female patients excluded