IN 1930 citrulline was discovered in water-melons1. Later it was shown to be an important participant in the Krebs and Henseleit ornithine cycle in animals and micro-organisms. In recent years several laboratories have worked on the ornithine cycle in plants. In this laboratory citrulline has been shown to play an important part in transporting and storing nitrogen in several families of higher plants2–4. Relatively high concentrations of citrulline in exuding sap and citrulline accumulated in artificially rooted leaves of alder trees show the root to have a determining influence in the biosynthesis of citrulline. However, excised branches of alder and hazel could also synthesize citrulline from ammonium nitrate under aseptic conditions. Though ornithine, the precursor of citrulline, is metabolized very quickly, we could not find significant amounts of it, but very high concentrations of the mono-N-acetyl-ornithine were found in all members of the family Fumariaceae, being characteristic of this family6. Ornithine reacting with carbamyl phosphate in the presence of carbamyl-transpherase is converted into citrulline. My experiments have shown this reaction to occur under physiological conditions even without enzyme, though converting carbamyl phosphate much more slowly to cyanate, which reacts with the amino-groups6,7. It was also possible to establish the biosynthesis of citrulline from ornithine and carbamyl phosphate in higher plants8–11.