[2-3H]Oripavine was administered to mature Papaver somniferum plants from two different chemical strains, one of which was known to contain oripavine. Morphinone was synthesized and used as carrier. Incorporation of oripavine into morphine was 28.1% and 21.4%, respectively. Morphinone decomposed during isolation, but the small amount isolated from one batch showed an incorporation of 4.5%. Thebaine and codeine were not radioactive. Therefore, the 3-O-demethylation of thebaine is not reversible. These results strongly suggest that oripavine represents a step on a second pathway from the baine to morphine.