Plants of the genus Securinega (family Euphorbiaceae) elaborate a group of unique tetracyclic alkaloids of which securinine (1) is the most abundant. 1 We wish to report experiments indicating that these alkaloids are formed by a novel biosynthetic pathway. Sufficient investigation of the biogenesis of piperidine alkaloids has been carried out to allow one to predict that the piperidine ring of 1 will be derived from L-lysine; 2 preliminary experiments in our laboratory support this prediction. Much greater interest surrounds the question of the origin of the remaining eight carbon atoms in the securinine skeleton. It occurred to us that this portion of the alkaloid might be derived from an aromatic amino-acid such as phenylalanine or tyrosine. This communication presents evidence that L-tyrosine, but not L-phenylalanine, serves as a specific precursor of securinine. Administration of DL-2-14 C-tyrosine to young Securinega suffruticosa (Pallas) Rehder plants by the cotton wick method gave after 14 days radioactive securinine (0.007% incorporation). 3 Administration of 'H(G)-L-phenylalanine to _S. suffruticosa under similar conditions yielded radioinactive securinine. This observation suggests that Securinega cannot convert L-phenylalanine into L-tyrosine by hydroxylation; this phenomenon has been observed in other Angiosperms. 2 The purified radioactive securinine obtained by