In order to elucidate the regulatory parameters which determine multiple opine formation in marine invertebrates, anaerobiosis was induced in 25 species from several phyla by stimulating the animals to vigorous muscular activity or by subjecting them to environmental hypoxia. The quantity of glycolytic end products and the corresponding amino acids were measured. In a second set of experiments the amounts of substrates and products of the opine dehydrogenase reactions in the isolated introvert retractor muscle (IRM) ofSipunculus nudus were determined in both situations.