Six calves were given dried, ground Cynoglossum officinale daily in a dose which provided 15 (two calves) or 60 (four calves) mg per kg per day of total pyrrolizidine alkaloids. Those calves given 60 mg per kg of total pyrrolizidine alkaloids per day died following a single dose of plant material. These calves had a marked elevation of serum gamma-glutamyltransferase (GGT) and aspartate transaminase (AST) activities and serum bile acid and total bilirubin (TBili) concentrations. These four calves all had massive hepatocellular necrosis and haemorrhage of the liver. Of the two calves that were given 15 mg per kg of total pyrrolizidine alkaloids per day, one died on day 34 and the other survived until day 35 when it was painlessly killed. There were significant elevations in serum AST and GGT activities in these calves. The histological lesions of the calf surviving until 35 days were compatible with pyrrolizidine alkaloid toxicity, that is megalocytosis, karyomegaly and necrosis of hepatocytes with karyomegaly of biliary epithelium. The pyrrolizidine base present in Cynoglossum officinale (heliotridine) and its esters have a similar type of toxicity to the highly toxic and more familiar macrocyclic diester pyrrolizidine alkaloids of the pyrrolizidine base (retronecine), present in Senecio or Crotolaria species.