The storage proteins of Vicia faba are characterized by a high proportion of arginine (Boulter & Davis 1968). The biosynthesis of arginine in plant tissues occurs by a pathway similar to the Krebs-Henscleit cycle of mammalian tissues (Reinbothe & Mothes 1962). During germination the storage proteins are hydrolyzed and the arginine released is metabolized mainly in situ (Jones & Boulter 1968). The present study deals with the subcellular distribution and the activity of two enzymes of this Krebs-Henseleit cycle from the cotyledons of broad beans during maturation and germination. One of the enzymes – ornithine carbamyltransferase – is concerned with the synthesis of arginine whereas arginase catalyses the breakdown of arginine in urea and ornithine. Extracts were prepared from cotyledons of broad beans germinated for 2 or 3 days. The extracts were fractionated by differential centrifugation. The fractions containing tightly coupled mitochondria showed hardly any ornithine carbamyltransferase activity but a very high arginase activity. The mitochondrial fraction was further fractionated on a sucrose gradient. The distribution of the arginase activity closely followed that of the succinate dehydrogenase activity. The arginase activity of the mitochondrial fraction was enhanced by freezing and thawing it by treatment with Triton X-100 or by exposure to an osmotic shock. These experiments clearly indicate that the arginase activity from broad bean cotyledons is located mainly in the mitochondria whereas ornithine carbamyltransferase activity is located outside the mitochondria. A quite different localization is found, however, in mammalian tissues and in Neurospora. Here, the arginase activity is located in the cytosol and ornithine carbamyltransferase activity in the mitochondria.