The administration of 5-oxooctanal-6-14C and 5-oxooctanoic-6-14C acid to hemlock (Conium maculaturn) resulted in the formation of coniine-1′-14C and γ-coniceine-1′-14C. The high incorporation of activity obtained with these compounds suggests that these are the immediate precursors of the alkaloids. The biochemical relationship of these precursors to other compounds (acetic-1- and -2-14C acid, hexanoic-1-14C acid, octanoic-1-, -7-, and -8-14C acid) which serve as precursors of coniine, and the pattern of labeling obtained with these compounds are rationalized. (+)-Coniine-2′-14C and (–)-coniine-2′-14C were fed to hemlock, and it was found that the natural (+) isomer was dehydrogenated to γ-coniceine much more efficiently than its enantiomorph. By administering 15N-abeled (+)-coniine and γ-coniceine to hemlock it was established that the nitrogen in these alkaloids is metabolically stable. A new alkaloid, (±)-conhydrinone (1′-oxoconiine), was isolated from hemlock, and its structure confirmed by synthesis. γ-Coniceine was shown to be a precursor of conhydrinone. © 1972, American Chemical Society. All rights reserved.