1. 1. The origin of carbon incorporated in succinate in the first 12 hr of environmental anoxia was studied in two bivalve species of the Northern Adriatic Sea. 2. 2. Posterior adductor muscles were excised after various periods of exposure to anoxic seawater and incubated under nitrogen in the presence of metabolic inhibitors (aminooxyacetate, iodoacetate, 3-mercaptopicolinate) and UL-14C substrates, in order to examine the redirection of glycolytic flux from pyruvate kinase to phosphoenolpyruvate carboxykinase. 3. 3. Changes in the levels of key metabolites and the distribution of radiolabeled glucose and aspartate indicate that PEP-carboxykinase did not participate in anaerobic metabolism of these species for at least the initial 12 hr of anaerobiosis.