Previous symposia on Metabolic Interconversion of Enzymes were held in 1970 (Santa Margherita-Ligure, Italy), 1971 (Rottach-Egern, Ger- many), 1973 (Seattle, USA) and 1975 (Arad, Israel). The present pub- lication reports the proceedings of the 5th International Symposium on Metabolic Interconversion of Enzymes, held in Titisee/Black Forest (Germany) from October 1st-5th, 1980. In the last few years, the number of enzymes for which control of activity by enzyme-catalyzed covalent modification, i.e., by intercon- version, is of recognized metabolic importance has increased so much that is was not possible to have every such enzyme considered during a 3-day conference. The organizers therefore decided to devote only one of the three sections to "metabolic interconversion" per se, and to cover in the other two sections "enzyme regulation by proteolytic modification" and "novel aspects of regulation". According to the IUP AC-IUB Commission on Biochemical Nomenclature (cf. J. Biol. Chem. 252,5939-5941 (1977)), modification by proteolysis is not in- cluded in "metabolic interconversion". Considering, however, the close interrelationship of these two types of enzyme control, it has become a tradition, beginning with the conference in Rottach-Egern 1971, to include proteolytic modification in our conferences.