The heat production rate of Gammarus oceanicus, a species of marine origin living in the brackish waters (7 psu (practical salinity units)), was examined by direct calorimetry after acclimation to salinities of 5, 7, 14, 20 and 30 psu (at 10 degreesC). The mean heat dissipation was greatest at 5 psu with 5.64 +/- 1.60 J h(-1) g(-1) ww (wet weight) and decreased significantly (P < 0.05) with increasing salinity down to 2.26 +/- 1.14 J h(-1) g(-1) ww at 30 psu. The high inter-individual variability was of significance for the obtained results. When salinity dropped by only 2 psu from the control (7 psu) the mean heat production rate raised by 18%. A salinity increase from 7 to 30 psu caused a reduction in the average metabolic rate of G. oceanicus by 53%. That means that G. oceanicus requires more energy for osmotic adjustment at lower salinities. (C) 2003 Elsevier B.V. All rights reserved.