Piperine is acutely toxic to mice, rats and hamsters. The LD50 values for a single i.v., i.p., s.c., i.g. and i.m. administration of piperine to adult male mice were 15.1, 43, 200, 330 and 400 mg/kg body wt, respectively. The i.p. LD50 value was increased to 60 mg/kg body wt in adult female and 132 mg/kg body wt in weanling male mice. In adult female rats, the i.p. LD50 value was 33.5 mg/kg body wt whereas the i.g. LD50 value was increased to 514 mg/kg body wt. Most animals given a lethal dose died of respiratory paralysis within 3-17 min. In subacute toxicity studies, the rats died within 1-3 days after treatment. Histopathologic changes included severe hemorrhagic necrosis and edema in gastrointestinal tract, urinary bladder and adrenal glands. Death of these animals may be attributable to multiple dysfunctions in their organs.