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.