The dihydropyrrolizine analogues (I) of the hepatotoxic pyrrolizidine alkaloids have been suggested as possible toxic metabolites of the alkaloids (1,2,3). The formation of Ehrlich-positive metabolites has been demonstrated (2,4) and the major metabolite of this type formed in the rat from heliotrine and lasiocarpine has been identified as dehydroheliotridine (II) (4,5). We describe here the preparation and properties of the dehydro-alkaloids (I), the metabolite (II) and related derivatives. The numbering system proposed for these compounds is based on a 5H-pyrrolizine system (IV) rather than the usual 3H-pyrrolizine (III) (6) in order to correspond with the parent alkaloids.