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Description / Table of Contents: Summary 1. The oxygen uptake of the euryhaline isopodIdotea balthica (Pallas), obtained from the Baltic Sea, was determined by means of flow-through polarography. The rates of respiration were studied in relation to body size, temperature and salinity. Measurements conducted at 5°, 10°, 15° and 20° C in 10 ‰ salinity revealed an almost continuous increase of metabolic rates with rising temperatures. The regression coefficients, calculated for size-dependent respiration, range from about 0.7 to 0.6. Compared with these data, a significantly higher regression coefficient was obtained from determinations made at 15° C and 30 ‰. The rates of active metabolism in adult individuals were shown to exceed standard metabolism by approximately 3 to 4 times at all temperatures tested. 2. The compensatory responses following thermal and salinity stress have been recorded in relation to the time courses of acclimation and the magnitudes of the physiological adjustments. Sudden alterations of temperature lead to new steady states of metabolic rates within 3 hours following a change from 15° to 5° C and approximately within 15 hours following a transfer from 5° to 15° C. At 15° C, an abrupt salinity change from 10 to 30 ‰ and vice versa requires a transition period between successive acclimation states of 6 hours (10 to 30 ‰) and about 30 hours (30 to 10 ‰), respectively. The transfer from the dilute to the more concentrated medium is accompanied by slightly reduced oxygen-uptake rates, whereas the transfer in the opposite direction leads to a marked temporary increase of respiration. 3. The time course and intensity of metabolic changes during moulting were also examined. The exuviations of the posterior and anterior body parts occur temporally separated and are reflected by two peaks of increased oxygen uptake, amounting to approximately three times the standard rates. In the interphase between the successive exuviations an elevated respiratory level is maintained. In adult specimens, the whole phase of increased metabolic requirements during moulting comprises a period from 40 up to 80 hours at 15° C. 4. The metabolic requirements and acclimatory responses ofIdotea balthica are considered in relation to its subtidal habitat and compared with compensatory reactions occurring in some intertidal and supratidal invertebrates. Further physiological and ecological implications are discussed.