OBJECTIVES: The authors have intended to know the drug interaction of fluoxetine and haloperidol when coadministering two drugs to the chronic schizophrenics by assessing the changes of positive, negative symptoms and extrapyramidal symptoms. METHOD: We selected 38 patients, the chronic schizophrenics with no physical problems. they are randomly assigned to placebo group and drug group. And then, placebo or fluoxetine 20mg were administered to the subjects of each group during 8 week period. We have assessed their psychopathology and extrapyramidal symptoms using positive and Negative Syndrome Scale(PANSS), Clinical Global Impression(CGI), Simpson-Angus Scale at o, 2, 4, 6, 8 week during the period. RESULTS: 38 patients have completed the study during 8 week. 1) PANSS, CGI : no significant difference between groups and no significant change according to the times. 2) Simpson-Angus Scale : no significant changes. CONCLUSION: When co-administering fluoxetine and haloperidol, there were no significant changes of psychopathology and extrapyramidal symptoms. There results suggest that it is safe to coadminister fluoxetine to schizophrenic with haloperidol treatment.