<jats:p>Intra- and extracellular acid-base status and changes of coelomic PCOCO2 were investigated during recovery following 24 h of anaerobiosis in Sipunculus nudus L. Metabolism, gas exchange and acid-base status were compared in animals collected during March and October.</jats:p> <jats:p>Anaerobiosis led to an uncompensated metabolic acidosis, the degree of the acidosis depending on the metabolic rate of the animals. During initial recovery in March animals, the acidosis was transiently aggravated in the extracellular, but not in the intracellular, compartment, indicating an efficient regulation of intracellular pH as soon as oxygen was available in the coelomic fluid. The extracellular acidosis was predominantly of non-respiratory origin.</jats:p> <jats:p>The non-respiratory part of the acidosis is attributed to the repletion of the phospho-l-arginine pool. The proton yield calculated from phosphagen resynthesis was highly correlated in time and in quantity to the observed negative base excess in the extracellular compartment. In October animals, strombine accumulation may have contributed to the acidosis that develops during recovery. The amount of succinate, propionate, and acetate in the coelomic plasma had already decreased when the acidosis developed. This discrepancy supports the conclusion that protons move between the body compartments independent of the distribution of anionic metabolites.</jats:p> <jats:p>The respiratory part of the acidosis is attributed to the repayment of an oxygen debt. The increase of PCOCO2 is higher in October than in March animals, probably because of differences in metabolic rate</jats:p> <jats:p>The time course of acid-base disturbances and their compensation is discussed in relation to the time course of metabolic events during recovery and to the priorities of the different processes observed.</jats:p>