Weight- and carbon-specific rates of respiration for 4 mesopelagic gelatinous zooplankton (2 ctenophores, 1 trachymedusa and 1 pelagic holothurian) measured in situ were 2 to 5 times higher than those from shipboard incubations. These results support the theory that removal of deep-living gelatinous species from natural environments can adversely affect their metabolism. The disparity between in situ and shipboard treatments is most simply explained by an interaction between pressure and activity, i.e. a loss of motor activity due to decompression occurred among animals used in shipboard experiments.