Abstract: Objective To investigate the roles of oxidative stress induced by paraquat treatment in the formation of severe of lethality in planarian Dugesia japonica. Methods The percentage of survival animals was used to reflect the lethality. The paraquat was used to induce the formation of severe oxidative stress, which was monitored by the superoxide dismutase (SOD) activity. DMSO, an effective free radical scavenger, was used to treat the planarians before or after the paraquat treatment. Results The significant decrease of SOD activity was observed in planarians exposed to 2 mmol • L-1 of paraquat for 1-h, and more severe decreases of SOD activity were further detected in planarians exposed to 2 mmol • L-1 of paraquat for 2, 4, and 8-h. After 2-h exposure, 1 mmol • L-1 of paraquat treatment did not induce an obvious alteration of SOD activity; however, the significant decreases of SOD activities were observed in paraquat exposed planarians at the concentrations from 2 mmol • L-1 to 4 mmol • L-1. Moreover, three days after the paraquat treatment, the significant decreases of percentages of survival animals were observed in planarians exposed to 2, 3, and 4 mmol • L-1 of paraquat. Furthermore, the significant increases of SOD activities were detected in 2 mmol • L-1 paraquat exposed planarians for 2, 4, and 8-h pre- or post-treated with 0.1% DMSO for 4-h compared with those in 2 mmol • L-1 paraquat exposed planarians for 2, 4, and 8-h, respectively. Nevertheless, the values of SOD activities in 2 mmol • L-1 paraquat exposed planarians for 2, 4, and 8-h pre- or post-treated with 0.1% DMSO for 4-h could not reach the value of SOD activity in planarians without paraquat treatment. The significant increase of percentage of survival animals was further observed in 2 mmol • L-1 paraquat exposed planarians for 2-h pre- or post-treated with 0.1% DMSO for 4-h compared with that in 2 mmol • L-1 p |