N-([4-¹⁴C]-4-Aminobutyl)-1,2-didehydropyrrolidinium chloride hydrochloride 19 was prepared from the mesylate of 3-chloropropanol by displacement of the mesylate with sodium [¹⁴C]cyanide, nucleophilic substitution of the chloro group by pyrrolidine, reduction of the nitrite and oxidation of the amine with mercuric acetate. This material was fed together with [2,3-³H]putrescine dihydrochloride to Senecio isatideus which produces retrorsine 6, Senecio pleistocephalus which makes rosmarinine 11, Cynoglossum officinale which contains echinatine 8 and Cynoglossum australe which produces cynaustraline 13 and cynaustine 15. Comparison of the ³H/¹⁴C ratios of the precursor mixtures with the alkaloids obtained after the feeding experiments had been carried out showed that the ¹⁴C-labelled immonium ion 19 is an efficient precursor for all the different base portions (necines) of the five pyrrolizidine alkaloids. The presence of the immonium ion 3 in Senecio pleistocephalus was established by an intermediate trapping experiment. Thus the immonium ion 3 is involved in the biosynthetic pathways to a range of necines, including retronecine 5, heliotridine 7, rosmarinecine 10, (+)-isoretronecanol 12 and (+)-supinidine 14.