Abstract: Objective To demonstr ate the effects of Na-DMPS on the LD₅₀ and pathological changes of the main tissues in mouse during acute toxicity of br omoxynil.Methods Using Bliss method to calculate the LD₅₀ of bromoxynil of the group of mice in acute bromoxynil poisoning and the group of mice in acute bromoxynil poisoning after Na-DMPS protection,and to observe the pathological changes of the main tissues of the two groups of mouse under optical microscope and electron microscope.Results The LD₅₀ of the group of mice in acute bromoxynil poisoning after Na-DMPS protection increased much more obviously than the LD₅₀ of the group of mice in aute bromoxynil poisoning(P<0.01).Under optical microscope:hyperemia in every lobe was obvious;there was swelling in brain,some colloid nodes could be seen in brain substance.But tissue damage in the group of mice in acute bromo xynil poisoning after Na-DMPS protection was not obvious.Under electron microscope,the group of mice in acute bromoxynil poisoning:dilation of endoplasmic retculum,balloning degeneration of mitochondrion and pyknosis were observed;there was swelling in endotheliocytes of lung;neural sheath was observed,nerve microfilament and microbule disappeared.A bout the guoup of mice in acute bromoxynil poisoning after Na-DMPS protection:dilation of endoplasmic reticulum in liver cell was observed;Edema could be seen in part of cerebral cortex.Conclusion The group of mice in acute bromoxynil poisoning shows multiple organ disfunction syndrome.The protection of Na-DMPS can increase the LD₅₀ of bromox ynil obviously and lessen pathological changes of the main tissues.