Terrestrial gastropods of the order Stylommatophora (Pulmonata) have a unique organization of ketone body metabolism, characterized by two cytosolic isoforms of the typically mitochondrial enzyme β-hydroxybutyrate dehydrogenase (BHBDH), which interconverts the ketone bodies acetoacetate and β-hydroxybutyrate (BHB). One of these cytosolic BHBDH isoforms is further unique in that it is specific for the L-enantiomer of BHB, whereas all other known forms of BHBDH oxidize exclusively D-BHB. The occurrence of the two cytosolic BHBDH isoforms is tissue specific. L-BHBDH is found in hepatopancreas and kidney, and D-BHBDH is found in heart and kidney. Enzyme activities suggest that L- and D-BHB may be synthesized through the incomplete oxidation of fatty acids in the kidney, where the highest activities of enzymes of lipid catabolism are found, and transported to peripheral tissues for oxidation. The modified BHBDH isoforms do not occur in the closely related pulmonate order Basommatophora, indicating that they have evolved only a single time, in the stylommatophoran lineage which successfully invaded terrestrial environments. The importance of these modified pathways of ketone body metabolism during estivation was investigated through measurements of maximal activities of ketogenic/ketolytic enzymes and those of carbohydrate, lipid and amino acid metabolism. No changes in the activities of any of these enzymes were observed after six weeks of estivation. Only the activities of citrate synthase and cytochrome c oxidase (CCO), which catalyze pathways which are "central" to aerobic metabolism, are reduced during estivation. The suppression of CCO activity may be mediated by changes in the phospholipid composition of mitochondrial membranes which occur during estivation. The content of cardiolipin, which is essential for maximal CCO activity is reduced by 83%. Mitochondrial membranes of estivating snails also contain fewer total phospholipids, fewer n-3 fatty acids and more monoenes, modifications which are consistent with reduced membrane proton permeability. Estivation induces other changes in the phospholipid and fatty acid composition of the mitochondria which are consistent with a more stable, less fluid membrane, suggesting reduced biological activity. Thus mitochondrial function is regulated during estivation and mitochondrial membranes appear to be an important target of this regulation.