Abstract: Objective To study the toxic effects and severity of ethaselen on beagle dogs.Methods The dosage of ethaselen, 600mg/kg, 200mg/kg and 67mg/kg were administered to three groups of beagle dogs separately.Control group was treated with solvent.The drug was given passes through the mouth for 12 successive weeks and 4 observing weeks.Then hematology, blood chemistry, electrocardiogram, urine, marrow, eyes, viscera dissecting, internal organs coefficient and histopathology were examined.Results After 12 weeks treatment, some dogs given high dosage of ethaselen showed symptoms of vomit, slight loss of feces colour, ALT and AST increase, glucose decline, the coefficient increase of liver, spleen and kidney and histopathology change of liver steatosis and adenomatoid prostate hypertrophy.All the changes basically disappeared after the drug withdrawal.Conclusions With equivalent to 42 times of clinic dosage of ethaselen, beagle dogs showed lightly symptoms such as alimentary tract stimulation, abnormal liver function, glucose level decline, the enlargement of liver, spleen and kidney, liver steatosis and adenomatoid prostate hypertrophy.There were toxic effects on alimentary tract, especially on liver.There were no apparently toxic effects to the beagle dogs at the dosage of 67mg/kg