In euryhaline osmoconformers like Arenicola marina the tissue cells are exposed to profound variations in osmotic concentrations. Aiming to increase our understanding of the underlying cellular adaptations, we measured the concentrations of total solute, electrolyte, chlorides and of small nitrogenous organic molecules (ninhydrin-positive substances, NPS) in the body wall tissues and in the coelomic fluid of specimens of Arenicola in response to sudden changes in salinity. The coelomic solutes consist almost entirely of electrolytes and the osmotic contribution of NPS is essentially negligible. In the body wall extracts, however, NPS accounts for at least one third of the osmotic concentration and for most of the substantial non-electrolyte fraction. There is no evidence from coelomic NPS measurements for extrusion of cellular amino acids during adaptation to lowered salinity. In diluted sea water the concentrations of the different solutes (including NPS) decrease by approximately similar rates and proportions, indicating responses to a common factor, such as tissue hydration. The concentration of chloride in the body wall tissues, however, showed distinctly greater decreases than that of other solutes, suggesting possible involvement of chloride compounds in the volume regulation of Arenicola.