Methods are described for the degradation of stylopine (2,3:9,10-bismethylenedioxyberbine)(6) which allow radioassay of various parts of the stylopine molecule. Samples of stylopine isolated from C. majus plants fed with a variety of multiply labelled precursors have been degraded by these methods, and these and other results show that (a)(+)-(S)-reticuline (4) is the precursor of (–)-(S)-stylopine (6); (b) C-1 and C-9 of reticuline are unaffected during its conversion into stylopine; (c) C-8 of stylopine is formed by oxidative cyclisation involving the N-methyl group of reticuline and this step involves an appreciable isotope effect; and (d) a small pool of scoulerine [as (5)] is present in C. majus plants, and the conversion of (–)-(S)-scoulerine (5) into (–)-(S)-stylopine (6) is demonstrated to occur in vivo without loss of tritium from the chiral centre. Some experiments on the rearrangement of N-oxides in vitro are described.