Parameters of acid-base and energy status were studied in three White Sea Littorina spp. (L. littorea, L. saxatilis and L. obtusata) during prolonged anaerobiosis in freshwater by in vivo 31P-NMR spectroscopy. Intracellular pH (pHi) decreased significantly, especially during the early period. Later on the pHi decrease slowed down considerably suggesting a good ability of pHi regulation in all three studied species. There was a trend for pHi to fall most rapidly in the least freshwater resistant species L. obtusata as compared to the most resistant L. littorea. Non-bicarbonate, non-phosphate buffer values estimated by the homogenate technique were similar in the three studied species (28 - 37 mmol pH-1 kg-1 wet weight) and did not change during freshwater exposure. The CaCO3 buffer value of the foot tissues was considerably higher (171-218 mmol pH-1 kg-1 wet weight) and decreased significantly during freshwater exposure. The contribution of the multiple tissue buffering systems to pHi regulation in Littorina spp. shifts between different stages of freshwater exposure. Initially the non-bicarbonate, non-phosphate tissue buffering system seems to be of major importance for metabolic proton buffering at pHi between 7.5 and 7.0. During later stages of anaerobiosis and at lower pHi, the CaCO3 buffer is involved in proton buffering. Decrease in the CaCO3 buffer value during freshwater exposure was in quantitative agreement with the amount of metabolic protons buffered thus suggesting that CaCO3 tissue stores may serve as a major buffering system during prolonged anaerobiosis in Littorina spp.