In order to explore the impact of mercury on fish, zebrafish were exposed in various levels of HgCl2 and the superoxide dismutase (SOD), acetylcholinesterase (AChE) activities were measured after 1 and 7 days exposure. Muscle RNA was also extracted for semi-quantitative RT-PCR determination of the expression of AChE, CYP1A1, Cu/Zn-SOD. It indicated that 96 h-LC50 of HgCl2 to zebrafish is 244.06 µg L(-1). The AChE activity was decreased on 1st day and 7th day. The SOD activity was almost unchanged on 1st day, it showed a dose-response effect on 7th day. The expression of AChE was down-regulated on 1st day, it is consistent with the AChE activity, but the expression was showed a significant difference between exposed group and the control group on 7th day. The expression of CYP1A1 was decreased compared with the control group on 1st day. After 7 day exposure, the expression of CYP1A1 was up-regulated in 0.1LC(50) group, and down-regulated in 0.4LC(50) group, 0.8LC(50) group, showing a stimulation effect at low concentration and inhibition at high concentration. The expression of Cu/Zn-SOD was down-regulated on 1st day, also showing an effect of stimulation at low concentration and inhibition at high concentrations. Thus HgCl2 was highly toxic to zebrafish, both the enzymatic activities and the gene transcription were changed significantly. But the change of gene transcription is earlier than enzymes activities. It indicates that gene transcript can be used as an early warning for environmental risk assessment.