Author information Laboratory of Experimental Pathology, National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709, USA. nyska@niehs.nih.gov Abstract Riddelliineis a naturally occurring pyrrolizidine alkaloid found in certain poisonous rangeland plants of the western United States. In National Toxicology Program 2-year studies, riddelliine induced high incidences of hemangiosarcoma in the liver of F344/N rats (both sexes) and B6C3F1 mic