In nature under adverse conditions like low oxygen conditions or starvation fish often lower their metabolism: 'metabolic depression'. This strategy of lowering the metabolic rate is a survival strategy and is used to save energy stores and diminish end-product accumulation. The overall metabolic rate of animals can be deduced by measuring metabolic processes such as oxygen consumption, but the ultimate method is measuring heat flow. In this review, we will summarise the available data about metabolic depression measuring heat flow, i.e. by direct calorimetry in fishes, which were carried out almost exclusively with a 1-I flow through calorimeter. Using deconvolution techniques the time constant of this calorimeter was measured that allowed to estimate the time course of metabolic depression, which was found to take place on a time scale of 20-30 min. We demonstrated that metabolic depression is species dependent. Goldfish, eel and tilapia show metabolic depression tinder low oxygen conditions while this is not the case for common carp. in addition it is shown that metabolic depression is flexible and increases with decreasing oxygen availability. Furthermore using a video analysing system we demonstrated that metabolic depression is not caused by a reduction of external activity. As heart rate falls dramatically during metabolic depression as shown by small wireless transmitters, we hypothesise that blood flow reduction might be the proximate cause for metabolic depression. (C) 2008 Elsevier B.V. All rights reserved.