The presence ofthe tricarboxylic-acid cycle (Krebs's cycle) has been demonstrated in most mammalian tissues investigated (Krebs & Johnson, 1937; Green, Loomis & Auerbach, 1948), where it appears to be the major pathway for the oxidation of carbohydrate, fat and protein intermediaries. No detailed examinations have been made in the invertebrates. However, evidence has been presented of Krebs's cycle activity in certain nematodes (Massey & Rogers, 1950), the cockroach (Barron & Tahmisian, 1948) and Drosophila (Spirtes, 1951). No similar studies have been made in Helix pomatia. The evidence of Biedermann (1911), Baldwin (1938) and Vonk, Mighorst & de Groot (1950) suggests that the hepatopancreas plays an important role in the metabolism of the animal. The enzyme systems examined in the present investigation were those contained in suspensions and mitochondrial preparations from the hepatopancreas of both hibernating and active H. pomatia, since there is evidence of changes in the enzymes of an organism during hibernation (Baldwin, 1938; Suomalainen, 1935). METHODS The snails used in these experiments were either kept in the refrigerator in the hibernating state, or had been active and fed on cabbage in the laboratory at 220 for at least 7 days before use. Unless otherwise specified, results reported in this paper were obtained with preparations from active H. pomatia. Enzyme preparations Suspensions. The hepatopancreas was rapidly removed and ground in an ice-cold Potter 'homogenizer' in either 0-2M-sucrose solution or 0 5 % KCl solution. The suspension was diluted with the appropriate medium to give a final concentration of approximately 20% tissue. Mitochondria. These were isolated according to the methods described by Schneider (1948) using 10% hepatopancreas suspensions made in 0 2M-sucrose. All preparations were carried out in a cold room at 20, a Servall SS-l-a high-speed centrifuge being used for the isolation of the mitochondria. Hexokinase. This was obtained by the method of Berger, Slein, Colowick & Cori (1946) as described by Cross, Covo, Taggart & Green (1949). The preparation was stored as a frozen solution at-100 and retained its activity for many months. The conditions for autolysis for baker's yeast were those described by Bailey & Webb (1948). Determinations Inorganic orthophosphate was determined by the method of Fiske & Subbarow (1925), and x-ketoglutarate and pyruvate by the method of Friedemann & Haugen (1943). Citrate was determined by the method of Natelson, Pincus & Lugovoy (1948) and sometimes by that of Weil-Malherbe & Bone (1949). Succinate was determined by the method of Krebs (1937). 02 uptakes were determined manometrically; the gas phase was air.