Abstract: The mechanism of anticonvulsant action of 7903 was studied. It may be related to serotoninergic system. 7903 was found to be able to increase the concentration of 5-HT and 5-HIAA in mice brain. The peak time of such action was at the first hour after treatment and gradually returned to normal level within 12 hours. This change of 5-HT was closely related to its anticonvulsant action, the correlation coefficent was 0.93 (P<0.001). Results of other experiments further support this finding. Reserpine was shown to antagonize the anticonvulsant action of 7903. Reserpinized mice were pretreated with L-tryptophane, the precursor of 5-HT, to supplement the concentration of depleted 5-HT (nearly to 80%).In such a condition, the anticonvulsant action of 7903 was recovered. In contrast, normal mice pretreated with PCPA to block the synthesis of 5-HT, decreased the anticonvulsant action of 7903.The concentration of NE was slightly decreased at the first hour after treatment with 7903, then gradually increased, and reached a peak on the 12th hours after treatment. At this time, the anticonvulsant action of the drug completely disappeared. The concentration of DA did not change within the 12-hour period.