In the Black Sea, during summer stratification, Calanus euxinus (Hulsemann) undertakes diel vertical migrations with an amplitude of about 117 m from oxygenated, warm (18 degreesC) surface layers to hypoxic (similar to0.8 mg O-2 l(-1)) zones with lower temperature (7.9 degreesC). When such changes in temperature and oxygen concentration are reproduced in the laboratory, total metabolism, basal metabolism and scope of activity of copepods decrease 7.2, 7.8 and 6.7 times, respectively, while the frequency of locomotory acts and mechanical power decline 3.4- and 9.5-fold, respectively. These changes allowed the copepods to conserve a significant portion of food consumed near the surface for transformation to lipid reset-yes. Diel respiratory oxygen consumption of migrating individuals, calculated so as to include actual duration of residence in layers with different temperature and oxygen concentrations, is estimated at 17.87 mug O-2 ind(-1) The net energy cost of vertical migration made up only 11.6% of the total. Copepods expend 78.6% of diel energy losses during approximately 10 h in the surface layers, while about 5.4% is required during about 9 h at depth Hypoxia is shown to have a significant metabolic advantage during diel vertical migrations of C. euxinus in the Black Sea.