Lytechinus variegatus were exposed to four salinity treatments (20, 25, 30 and 35 ppt S) for 32 days. All compartmental fluids showed osmoconformity at all salinities. Sodium levels were slightly, but significantly, higher in the lumenal fluids of the stomach and intestine as compared to the external medium at all salinities. Chloride levels were significantly lower in the coelomic fluid in comparison to the seawater and the stomach and intestinal lumenal fluids. We hypothesize that chloride moves from the lumen of the stomach and intestine into the coelom, then into the external medium, generating an ion flux aiding in the co-transport of nutrients or other molecules. Potassium levels in the lumen of the stomach and intestine remained significantly higher and similar in all salinity treatments, while potassium levels in other compartment fluids varied directly with salinity. At low salinity, relatively high stomach and intestine lumenal fluid levels of potassium may represent a stress response and ultimately reflect physiological mechanisms leading to the death of the individual. © 1994.