Albizziine (2-amino-3-ureidopropanoic acid) was identified as a major radioactive metabolite of [1,2-¹⁴C] uracil in tissues and enzymic extracts of seedlings of Albizzia julibrissin. Consistent with the biogenesis of albizziine via uracil catabolism was the observation that [1,2-¹⁴C]uracil is incorporated into isobarbituric acid (2,4,5-trihydroxypyrimidine) by crude enzymic extracts and microsomal preparations of seedlings of A. julibrissin and Pisum sativum in a tetrahydropterin-dependent hydroxylation of uracil. Substantial stimulations, more than 500% above the control value with A. julibrissin, were seen on addition of 6-methyl-5,6,7,8-tetrahydropterin. That there is a direct precursor relationship between uracil and albizziine was confirmed by the specific incorporation of the ¹⁴C-label from [2-¹⁴C]uracil into the ureido-carbonyl of albizziine. This is as would be expected for the ring opening of a dihydropyrimidine, catalysed by dihydropyrimidinase. As 5-aminouracil had previously been identified as a precursor of albizziine, it was concluded from the present work that the biogenetic sequence from uracil involves first, hydroxylation to isobarbituric acid, then amination to 5-aminouracil, followed by hydrogenation and ring-opening, to yield albizziine, from which 2,3-diaminopropanoic acid is formed by the action of β-ureidopropionase. The formation of albizziine and 2,3-diaminopropanoic acid represents a further example of the interfacing of pyrimidine primary and secondary metabolism through uracil.