The mechanisms of frahmentation of retusine,spectabiline,usaramine and diacetyl-monocrotaline were stL died by means of exact mass measurements and metastable ion measurements and also by using corresponding compounds labeled with deuterium.Significant difference was found betv-oen the mechanisms of fragmentation of maorocyclic dilactone pyrrolizidine alkaloids with and without a double bond in the fused ring system.In the latter case,the cleavage of the C₇-O bond is preferred,whereas in the former case,due to the presence of the allylic oyygen function,the C₉-O bond is more readily brokon.