Intra- and extracellular acid-base status was investigated during prolonged experimental anaerobiosis in Sipunculus nudus L. An acidosis could not be observed during the first 6-12 h of anaerobiosis, in contrast, a slight alkalosis developed in both extra- and intracellular body compartments. Extra- and intracellular pH only started to decrease gradually after 12 h of environmental hypoxia as an expression of a non-compensated non-respiratory acidosis. The initial alkalosis associated with a positive base excess is interpreted as being due to the concomitant degradation of phospho-l-arginine (Pörtner et al. 1984a). The amount of succinate, propionate, and acetate accumulated in the extracellular fluid (coelomic plasma) could not be correlated quantitatively with a concomitant negative base excess. This discrepancy suggests that protons and anionic metabolites are distributed between various body compartments according to different equilibria and kinetics. Comparison of the changes in the acid-base status with the concentration changes of characteristic anaerobic metabolites (Pörtner et al. 1984a) indicates that (at least in Sipunculus nudus) pHi is not the crucial factor initiating the observed shift of the metabolite flux from the Embden-Meyerhofpathway towards the succinate-propionate pathway. © 1984 Springer-Verlag.