Pyrrolizidine alkaloids (PA's) are a group of plant secondary compounds which have received much attention in plant-insect relationships (for complete refs: [1, 2]). Although produced by plants as protective agents against herbivory, some plant feeders have evolved to cope with these compounds or even to use them for their own benefit. To give an example, larvae of the European cinnabar moth, Thyria jacobaeae (Arctiidae) [3], or grasshoppers of the genus Zonocerus [4, 5] accumulate PA's of plant origin for their own protection. Some Lepidoptera even use PA's as precursors for pheromone biosynthesis [1, 2]. On the other hand, larvae of the noctuid moths Spodoptera littoralis and Melanchra persicaria readily feed on Senecio vulgaris, a rich source of PA's, without storing detectable amounts of PA's in their bodies [3]. It is still unknown whether or how PA's, that are taken up by such polyphagous insects, are detoxified. In Senecio vulgaris PA's are synthesized, translocated, and accumulated as alkaloid N-oxides exclusively [6-11]. The migratory grasshopper Melanoplus sanguinipes, a major pest of North American rangelands and cereal crops, readily feeds on S. vulgaris. The fate of PA's in this insect was studied, using 14C-labeled senecionine N-oxide and senecionine, prepared biosynthetically from [1,4-14C]putrescine [6]. For comparison, N-methyl-3H-labeled atropine and its Noxide were included in the studies.