Dopamine has been accepted as a possible etiological factor in schizophrenia and most studies have demonstrated that the ingestion of methionine by schizophrenics exacerbated their psychosis. Unfortunately, the methylation theory has failed to support the dopamine theory in schizophrenia. This paper will attempt to demonstrate why abnormal histamine metabolism in schizophrenia can explain the effect of methionine in worsening schizophrenia in certain patients.