The ability of blue mussels Mytilus edulis L. to withstand severe environmental hypoxia was studied in mussels from an intertidal population and from a suspended cultured settlement. Specimens were exposed to air for 60 h at +10°C. Tissues were analysed for the amount of anaerobic metabolic end products, adenylates, phosphagen, and inorganic phosphate, and for changes in intracellular pH (pH(i)). Proton balance and Gibb's free energy of ATP hydrolysis were calculated. Under the experimental conditions applied succinate appeared to be the only anaerobic end product. Under control conditions pH(i) measured using the homogenate technique ranged between 6.78 and 6.85 in both groups. Very small decreases in pH(i) were observed after air exposure. Rapid breakdown of ATP and phospho-L-arginine (PLA) was accompanied by the accumulation of inorganic phosphate, free AMP and ADP. The Gibb's free energy change of ATP hydrolysis decreased from about -57 to -50 kJ mol-1, showing the depletion of energy reserves in the tissues. The calculated ATP turnover rate was higher in intertidal mussels. It is concluded that mussels from the intertidal zone demonstrate lower abilities for metabolic depression under the conditions of air exposure than cultured (sublittoral) ones. The latter are characterised by higher initial PLA content. These differences may be related to a difference in mitochondrial density depending on the habitat.