Technical Abstract: Since the early 1900’s, Baccharis pteronioides DC has been intermittently associated with livestock poisoning in the southwestern United States. In 2004, nearly 100 free ranging cows were reported poisoned by B. pteronioides in southern New Mexico. Initial field studies and post mortem examinations found drought conditions, evidence of B. pteronioides consumption, and a reported mortality of nearly 40%. As post mortem materials were unsuitable for further examination, plant samples were collected for feeding trials and chemical evaluation. Forty-eight, 8-week-old Syrian hamsters were randomly divided into 4 groups and dosed with 0, 50, 100 and 200 mg of finely ground, freeze-dried B. pteronioides for 10 days. After dosing, the hamsters were necropsied; sera were analyzed biochemically; and tissues were collected and evaluated histologically. The hamsters treated with 200 mg and several of the 100 mg animals developed anorexia and diarrhea. These animals developed multiple hemorrhagic infarcts in the liver and kidney with severe hemorrhagic enteritis. Histologically the poisoned animals had severe necrotizing vasculitis with vascular thrombosis of hepatic and renal vessels. Many glomerular capillaries contained fibrin thrombi. The superficial intestinal and colonic mucosa had necrotic with extensive hemorrhage and proliferation of luminal bacteria. Lower dose animals had mild hepatocellular swelling with proliferation of intestinal and gastric bacteria and yeast. These findings indicate that at high doses, B. pteronioides is toxic to hamsters and produces lesions that are very similar to bacterial endotoxin produced vasculitis and infarction. Research to purify and identify the toxin, the toxic dose and mechanism of toxicity are ongoing.