The scope of anaerobic metabolism of Sipunculus nudus L. was assessed from the maximal activities of some enzymes of the intermediary metabolism and from the concentration of some metabolites accumulated during enhanced muscular activity and during prolonged experimental hypoxia. (1) Maximal enzyme activities demonstrate that the scope of anaerobic glycolysis, as indicated by maximal activities of glycogen phosphorylase (0.84 U g-1 fresh wt), far exceeds the aerobic capacity, which is assumed not to surpass the activity of succinate dehydrogenase (0.09 U g-1 fresh wt). Three pyruvate reductase activities (alanopine-, strombine- and octopine dehydrogenase) can possibly terminate anaerobic glycolysis. (2) During muscular activity, energy is provided by the degradation of phospho-L-arginine and by anaerobic glycolysis. Octopine is the major endproduct during functional anaerobiosis while the formation of strombine is less pronounced. (3) During exposure to a nitrogen atmosphere, several anaerobic endproducts are found to accumulate. Anaerobic glycolysis is terminated by strombine synthesis. This opine accumulates in concentrations much higher than octopine. In addition the concentrations of succinate, propionate and acetate are found to increase in tissues, and/or in the coelomic fluid and the incubation water. (4) The relative contribution of energy by the different anaerobic metabolic pathways are estimated during functional and environmental hypoxia. © 1984 Springer-Verlag.