There is strong evidence that oxygen free radicals may play an important role in the pathophysiology of schizophrenia. Impaired antioxidant defense and increased lipid peroxidation have been previously reported in drug-naïve, first episode and chronically medicated schizophrenic patients using typical neuroleptics. We measured serum SOD and TBARS in two groups of chronic medicated DSM-IV schizophrenic patients, under haloperidol (n = 10) or clozapine (n = 7) and a group of healthy controls (n = 15). Serum SOD and TBARS were significantly higher (p = 0.001) in schizophrenic patients (7.1 +/- 3.0 and 3.8 +/- 0.8) than in controls (4.0 +/- 1.6 and 2.5 +/- 0.7). Among patients, serum TBARS was significantly higher (p = 0.008) in those under clozapine (4.4 +/- 0.7) than in those under haloperidol treatment (3.4 +/- 0.7). For SOD levels the difference between groups was not found. It is reasonable to argue that the difference found in TBARS levels might be due to the course of the disease, instead of medication. Further investigation on the role of oxidative tonus and lipid peroxidation in different schizophrenia subtypes and different outcome patterns in this disorder is warranted. Additionally it could also address questions concerning a possible oxidant/antioxidant imbalance in schizophrenia.