<jats:p>Proton-equivalent ion transfer processes between animals and ambient water were determined under normoxic control conditions during anaerobiosis and the subsequent recovery period in the marine worm Sipunculus nudus L. During anaerobiosis and recovery, transepithelial H+-equivalent ion transfer was generally correlated with changes in extracellular pH, with some disparities in ‘spring’ animals. The typical initial alkalosis induced by phosphagen cleavage during early anaerobiosis was reflected by a loss of basic equivalents. The acidosis, which developed later, reflecting production of acidic metabolic intermediates, resulted in a relatively small net extrusion of protons into the water. The coelomic acidosis during recovery was greatly exaggerated by the release of protons during phosphagen repletion and by the considerable elevation of Pco2 after normoxia had been reattained. The acidosis stimulated the net release of H+ to the water at a rate several times higher than that during anaerobiosis. The efficient transfer of protons from the body fluids to the environmental water during recovery facilitated normalization of coelomic pH, long before protons dissociated from the large amounts of organic acids produced as anaerobic intermediates could be removed from the body fluids by metabolism.</jats:p> <jats:p>Although the transfer of net H+ equivalents to the water coincided with coelomic acidosis, the rates of transfer during different periods of the experiment were primarily correlated with overall metabolic rate. Low net proton transfer rates associated with anaerobiosis were not sufficient to maintain acid-base parameters typical for normoxia, whereas re-establishment of aerobic conditions facilitated a greatly increased transepithelial H+ transfer rate. These data suggest that the transfer capacity of the energy-consuming translocation mechanism may primarily be determined by the rate of metabolic turnover and, accordingly, by theamount of available energy.</jats:p>