A sample of (2R) - [2-2H] putrescine (13) dihydrochloride was prepared from (2s)-aspartic acid (8), and (2s) - [2-2H] putrescine (15) dihydrochloride was synthesized from (2R)-aspartic acid. Feeding experiments carried out with these precursors on Senecio isatideus plants gave retrorsine (5) containing 2H, and the distribution of 2H from each experiment in retrorsine was determined by 2H n.m.r. spectroscopy. All of the 2H was confined to the base component of the alkaloid, retronecine (4). Retrorsine (14), derived biosynthetically from (2R) - [2-2H] putrescine (13) dihydrochloride was labelled with 2H at C-2 and C-6α, while retrorsine (16), produced from (2s) - [2-2H] putrescine (15) dihydrochloride contained 2H labels at C-6β and C-7α. These labelling patterns demonstrate that hydroxylation at C-7 of retronecine (4) proceeds with retention of configuration. In addition, the formation of the 1,2-double bond of retronecine involves removal of the pro-S hydrogen and retention of the pro-R hydrogen at the carbon atom which becomes C-2 of retronecine. © 1987, The Royal Society of Chemistry. All rights reserved.