The mode of incorporation of [1,2-¹³C₂]cadaverine (2) dihydrochloride into (-)-N-methylcytisine (14) in Baptisia australis was established by ¹³C n.m.r. spectroscopy. The tricyclic alkaloid (-)-N-methylcytisine displayed a labelling pattern (16) which was consistent with degradation of a tetracyclic intermediate. The (R)-[1-²H]- (11) and (S)-[1-²H]-cadaverine (12) dihydrochlorides were prepared and fed to Baptisia australis, and labelling patterns in the alkaloids were determined by ²H n.m.r. spectroscopy. After feeding the (R)-precursor (11), ²H was retained in (+)-sparteine (13) at C-2α, -6β, -11β, -15α, and -17β, whereas C-2β, -10β, and -15β were labelled from the (S)-precursor (12). The presence of ²H at C-17β in (+)-sparteine after feeding the (R)-isomer (11) shows that 17-oxosparteine [enantiomer of (10)] cannot be an intermediate in the formation of (+)-sparteine. With (-)-N-methylcytisine (14), ²H was retained after feeding the (R)-isomer (11) at C-10β and C-11β; the (S)-isomer (12) labelled C-13β. Comparison of these labelling patterns (19) with those of (+)-sparteine (17) establishes which outer ring of a tetracyclic intermediate is cleaved and which is converted into a pyridone during the formation of (-)-N-methylcytisine (14).