Extracts of annual ryegrass. (Lolium rigidum) infected with Anguina sp., and Corynebacterium sp. and associated with an outbreak of annual ryegrass toxicity in sheep, were administered to 2-week-old rats by a single intraperitoneal injection. Rats that received a lethal dose of toxin developed neurological signs including incoordination and convulsions from the second day and most died between 2 and 7 days after injection. Histologically, the brains showed widespread lesions of focal necrosis consistent with anoxia. Peripheral circulation rate was greatly reduced after 3 days and gangrene of the tail and hind legs developed in some rats, Evidence of restricted blond How was also seen in kidneys, lungs and bruin. It was concluded that the toxin contains a long-acting vasoconstrictor.