This study was conducted to determine the safety of bensulfuron-methyl and mefenacet in rice ecosystem. A field experiment was carried to reveal residual dynamics and final residue of bensulfuron-methyl.mefenacet 4.2 g/kg granule in rice and paddy system in Hangzhou, Changsha and Nanning of China during 2010 to 2011. The results showed that the average recoveries of bensulfuron-methyl and mefenacet spiked in the paddy soil, paddy water, rice, husk and rice plant at the 3 concentration levels of 0.05, 0.10 and 1.00 mg/kg ranged from 70.78% to 116.06% with the relative standard deviations of 0.91%-10.24%. The limit of detection （LOD） values of bensulfuron-methyl and mefenacet were 0.02 mg/L, and the minimum de- tection quantities were 4.0×10-9 g. The degradation dynamics test was done by spraying at a high dose （270 kg/hm2, bensulfuron-methyl active ingredient was 64.8 g/hm2, mefenacet active ingredient was 1 069.2 g/hm2） by direct spreading method 5-7 d after transplanting, and the final residual test carried out at a low dose （180 kg/hm2, bensuifuron-methyl active ingredient was 43.2 g/hm2, mefenacet active in- gredient was 712.8 g/hm2） and a high dose （270 kg/hm2）. The degradation research of bensulfuron-methyl and mefenacet in paddy water, soil and rice plant suggested that the degradation curves accorded with the first-order kinetics equation, the aver- age half life of bensulfuron-methyl was 5.35, 3.05 and 3.71 d in water, soil and rice plant, respectively, and the average half life of mefenacet was 3.61, 3.29 and 3.88 d in water, soil and rice plant, respectively. The final residues of bensulfuron-methyl and mefenacet were not detected in normal harvest paddy soil, rice straw, husk and brown rice.
