The incorporation of (±)-N-norprotosinomenine, (±)-N-nororientaline, (±)-N-nor-reticuline, (±)-norlaudanosoline, (±)-protosinomenine, and N-[2-(3-hydroxy-4-methoxyphenyl)ethyl]-2-(4-hydroxyphenyl)ethylamine into cocculidine has been studied, and the specific utilization of the (±)-N-norprotosinomenine demonstrated. A double labelling experiment with (±)-[1-³H, 4′-methoxy-¹⁴C]-N-norprotosinomenine showed that the 4′-O-methyl group of the precursor is retained in the bioconversion and the erythrinan ring system is not formed by addition of the secondary amino function onto an ortho-quinone system. Parallel experiments with (+)- and (–)-N-norprotosinomenine demonstrated specific incorporation of the (+)-isomer into cocculidine. High incorporation of cocculidine into cocculine revealed that O-demethylation is the terminal step in the biosynthesis of the latter. Feeding experiments also revealed that the plants can convert isococculidine into cocculidine with very high efficiency.