A fully automated enzyme immunoassay system, Omni (BIO-TEK Co.) (ELISA system), using Markit-M Excegran and Markit-M Haloperidol (Dainippon Pharmaceutical Co.) was evaluated to determine the concentrations of zonisamide (ZNS) and haloperidol (HP) in human plasma. At first, the within-run (ZNS : n=8, HP : n=10) and the between-run (4 days) precisions of ZNS and HP were examined. The coefficient of the variations in both the within-run and the between-run previsions of HP were below 5.0 and 7.00/0, respectively. In contrast, the some values of ZNS were below 6.5 and 8.40/0, respectively. A good correlation was observed between this ELISA system and HPLC (r=0.917), which was used to determine the plasma ZNS concentration. In the case of HP, a good correlation was seen between this ELISA system and the manual ELISA method (r=0.978). This method was thus used to monitor the serum levels in patients receiving ZNS or HP therapy. The relationship was observed between the daily dose (mg/kg) and the plasma concentration of the trough level for ZNS and HP, respectively (r=0.750 p<0.001, r= 0.669 p<0.001). However, the ratio of the plasma concentration of ZNS and the daily dose increased significantly with aging.