Low rates of pyrrole metabolite production coupled with glutathione conjugation in sheep may explain the resistance of sheep to senecionine. High rates of GSH-DHP conjugation appears to contributing to the resistance of hamsters Huan 1998 . The lipophilic tertiary alkaloid is absorbed passively and then bioactivated by cytochrome P-450 oxidase Lindigkeit 1997 . Castagnoli 1997 . The N-oxide was converted to both dehydroretronecine and the parent alkaloid Couet 1996 .
Pyrrolizidine alkaloids obstruct the hepatic venous system and can lead to hepatonecrosis.
Clinical manifestations include abdominal pain, ascitis, hepatomegaly and raised serum transaminase levels. Poisoning is related to the roots in spring Larrey 1994 . Although pyrrolizidines are well studied, hepatotoxic effects of comfrey alkaloids are scant. Yeong 1993 . Winship 1991 .