Abstract: Artemether (Dihydro-artemisinine-12-β-methyl ether), an ether derivative of reduced qinghaosu, has been found to possess schistosomicidal actions.Mice, rabbits and dogs were exposed to 50±2 and 250±2 of Schistosoma japonicum cercariae. When infected mice were treated orally with artemether suspension at the dosages of 400～800 mg/kg in 1～4 day-course, the worm reduction rates were 55.3%～79.9%. If the drug in oil formulation was given subcutaneously, to infected mice at the dosages of 225～435 mg/kg in 3 day-course, the worm reduction rates were 70.5%～81.2%. In infected dogs treated orally with artemether suspension at the dosages of 25～35 mg/kg in 3 day-course, or treated intramuscularly with the drug in oil formulation at the dosages of 150～250 mg/kg in 5 day-course, the worm reduction rates were 52.6%～59.1% and 91.3%～99.3% respectively. Artemether was also effective against immature worms.When both infected and normal dogs were treated intramuscularly with artemether in oil formulation at the dosages of 30～90mg/kg/day for 5 days, anorexia and diarrhea occurred in some dogs, and subsided 4～5 days after the end of treatment. Three days after the end of medication, the average white blood cells were markedly decreased in uninfected dogs, but returned to normal 18 days after medication.Histo-patholgical changes of the normal dogs treated with artemether were examined. It was domonstrated that degeneration of mucosa of stomach and large intestine, villi of small intestine and epithelium of renal tubule; dilatation of blood vessels and retention of plasma in glomerulus; and fatty degeneration in liver and in adrenal were found. The above mentioned tissue alterations nearly recovered to normal 18～19 days or 30～31 days after the cessation of treatment. No significant changes were found in other tissues of the animals.