This study was designed to compare the [123]I-CNS 1261 binding to NMDA receptor between healthy volunteers and different subgroups of schizophrenic patients. Investigation of the potential influence of antipsychotic and concomitant medication on [123I] CNS 1261 binding is also relevant.Fifteen healthy subjects (male and female of non-child bearing potential) will be recruited and 40 schizophrenic patients divided in 3 subgroups as indicated before: Subgroup a) treatment-naive, (n=10); Subgroup b) on stable treatment with risperidone (without criteria of deficit syndrome on the SDS scale, (n=15);Subgroup c) on stable treatment with risperidone fitting criteria of deficit syndrome on the SDS scale, (n=15).