1,2-Dehydropyrrolizidine derivatives, mainly the hepatotoxic alkaloids and their amino alcohols, are converted into 6,7-dihydro-5H-pyrrolizine derivatives. The dehydro derivatives of the amino alcohols are prepared with chloranil. Oxidation by manganese dioxide or potassium permanganate, or catalytic dehydrogenation, also leads to the desired dihydropyrrolizines together with products at a higher oxidation level such as 7-oxo and 1-formyl derivatives. The dehydro derivatives of the ester alkaloids are best prepared by the action of acetic anhydride on the N-oxides. The reactions of the dihydropyrrolizines with alcohols and amines, and their N.M.R. spectra, are discussed.