The incorporation of (+)- and (–)-N-norprotosinomenine into erythraline and erythratine in Erythrina crista galli has been investigated and the specific utilisation of the (+)-isomer has been demonstrated. Double-labelling experiments have shown that in the same system 5,6,8,9-tetrahydro-2,12-dimethoxy-7H-dibenz[d,f] azonine-3,11-diol and erysodienone are also genuine precursors. The nature of the symmetrical intermediate shown to be involved in the conversion of (+)-N-norprotosinomenine into (5S)-erysodienone, is discussed. Later stages in the biosynthesis of erythraline have been investigated and shown to follow the lines previously postulated. The point of formation of the methylenedioxy-group is apparently not critical.