Abstract: The acute oral LD50 of tribendimidin in normal mice and rats were 950±207mg/kg and 2 001 ± 79mg/kg, respectively. Its LD50 by peritoneal injection in mice was 277 ± 27mg/kg. The ED50 of the drug in rats infected with Nippostrongylus braziliensis, as estimated in previous studies was 5.1mg/kg. The therapeutic index and safety index were 394 and 48mg/kg respectively.When rats and dogs were orally administered with tribendimidin at the dosage of 125-750 and 30-120mg/kg/d, respectively for 14 days, no change in the liver and renal functions and blood or urine routine examinations was observed. The histopathological lesions in liver, stomach, small intestinal and kidney were mild and reversible. Salivation, vomiting and intermittent convulsion and even death due to respiratory failure, were found in dogs receiving 5-20 times the recommended clinical oral dosage of tribendimidin.