The Purposes of this study were to examine plasma homovanillic acid(pHVA) levels and 5-hydroxyindoleacetic acid(pHIAA) levels in schizophrenics during haloperidol treatment, and to assess the association of pHVA and pHIM levels with their psychopathology and treatment responses. Fourteen patients entered the study and pHVA, pHIAA levels were measured at baseline, first week, second week and fourth week during treatment. Also, plasma haloperidol levels were measured at first week, second week and fourth week. Psychopathology was evaluated with Brief Psychiatric Rating Scale(BPRS) at baseline, 1st week, 2nd week and 4th week. 1) There were significant differences on the duration of illness and total BPRS scores at baseline between higher pHVA group(baseline pHVA level >7.72ng/mL) and lower pHVA group(baseline pHVA level <7.72ng/mL). 2) There was no significant difference on the duration of illness between higher pHIM group(baseline pHIAA level >3.18ng/mL). and lower pHIAA group(baseline pHIAA level <3.18ng/mL). 3) The Means of pHVA levels at 1 st week and 2nd week after treatment decreased significantly in the higher pHVA group and did not change in the lower pHVA group. 4) In the higher pHIAA group, the mean of pHIAA levels at 4th week after treatment decreased significantly, but did not change in the lower pHIAA group. 5) Between the higher pHIVA group and lower pHVA group, the response rates(percentile improvement) after treatment were not different from each other, but there was significant difference on the response rate between the lower pHIAA group and higher pHIM group at 2nd week. 6) There was significant correlation between total BPRS scores and pHVA levels in the higher pHVA group during treatment. The results suggest that repeated measurement of pHVA levels and pHIAA levels following antipsychotic treatment have prognostic significance for response. Also, shcizophrenics whose have relatively nigh levels of pHVA, or relatively low levels of pHIAA before treatment will show a favorable early responses to antipsychotics.