Anaerobic metabolism and changes in the osmotic concentration of extravisceral fluid were studied in the White Sea periwinkles (Littorina littorea, Littorina saxatilis and Littorina obtusata) during freshwater exposure. Resistance to hypoosmotic stress increased in the order: L. obtusata < L. saxatilis < L. littorea. Our data suggest that osmotic shock is not a primary reason for mortality of the periwinkles under these conditions. During environmental anaerobiosis, considerable succinate accumulation (up to 10-19 μmol g-1 wet weight), and depletion of phosphagen and ATP pools were found in the studied species. Other metabolic end products (alanopine, strombine, lactate, acetate or propionate) were not detected. Succinate accumulation and net ATP breakdown were the fastest in the least resistant species, L. obtusata, and slowest in the most resistant, L. littorea. Rate of ATP turnover decreased during freshwater exposure in L. littorea and L. saxatilis, but not in L. obtusata. Our data suggest that differential resistance of three studied Littorina spp. to extreme hypoosmotic stress may be related to their different abilities to reduce metabolic rate and ATP turnover during sustained anoxia. Species-specific variations in anaerobic capacity of Littorina spp. are discussed in relation to their vertical distribution, size and ecology.