Feeding experiments with 14C-labelled reticuline, protosinomenine, orientaline, their N-nor-analogues and 3H-labelled isoboldine have shown reticuline and isoboldine to be the most efficient precursors of thalicarpine in Thalictrum minus. A biosynthetic pathway for thalicarpine with reticuline and isoboldine at the benzylisoquinoline and aporphine stages respectively has been suggested. Support for this proposal has been provided by the demonstration by radioisotopic dilution that reticuline and isoboldine are minor constituents of the plant. © 1982.