Seawater salinity changes affected the metabolic rates of the sympagic (under-ice) amphipod Gammarus wilkitzkii from the Arctic. When amphipods were abruptly transferred from seawater of 35 'L to 5%. the rates of both standard specific oxygen consumption (2.31 f 0.89 p1 g-' dry wt min-') and ammonia excretion (0.97 2 0.17 yg-at. NH4-N g-l d. wt h-l) increased about 3 times during the first hours of the exposure and remained hlgh throughout the rest of the experimental period (72 h). However, metabolic rates appeared to be less sensitive to 24 h osmotic stress in the salinity range 25 to 48%. compared to exposures of 15 and 5 %. The 0 :N atomic ratio of about 21 was constant during hyposmotic stress, indicating protein/lipids as metabolic substrate. Rates of oxygen consumption and ammonia excretion both increased with increasing osmotic difference (0 to 450 mOsm) between the haemolymph and the medium, indicating higher energy requirements for osmotic and ionic regulation at low salinities. A minor decrease in haemolymph sodium concentrations in amphipods exposed abruptly to 5%" coincided with an increased ammonia output during hyposmotic stress, indicating possible counter-ion regulation of NH,' and Na+.