In animals, various organic acids are accumulated during hypoxia or anoxia as products of anaerobic energy metabolism. The diversity of such acids is largest in marine invertebrates where succinate, propionate, acetate, lactate, alanine, octopine, strombine, and alanopine, are produced mainly from glycogen and aspartate. The effect of these substances on the acid-base status was assessed by a theoretical analysis of the respective metabolic pathways. This resulted in a general rule which was applied to evaluate the proton balance of the reactions in energy metabolism: net changes in the number of carboxyl groups or changes in the degree of dissociation of other groups (e.g. phosphate or ammonia) determine the net amount of H+ ions released or bound by the substrates and the metabolic end products. For marine invertebrates the results of the analysis can be summarized as follows: In glycogenolysis one mol of protons per mol of end products is released during cytosolic glycolysis, independent of the type of metabolic end product (lactate, octopine, alanopine, strombine, or alanine). The same applies for mitochondrial production of propionate and acetate, whereas formation of succinate results in dissociation of two mol H+ per mol. Fermentation of aspartate, however, diminishes the amount of protons which is produced in the succinate-propionate pathway. Net metabolisation of Mg ATP2- yields extra protons, whereas the cleavage of phosphagens (e.g. creatine phosphate, arginine phosphate) consumes protons. Additionally the break-down of energy-rich phosphates to inorganic phosphate has to be taken into account because of the shift of the intracellular buffer curve caused by changes of the respective effective pK values. © 1984 Springer-Verlag.