The measurement of heat production represents the most general approach to the estimation of energy flow through biological systems. While aerobic energy metabolism is most conveniently studied by polarographic oxygen determination, direct calorimetry presents the only unspecific method for quantitative comparison of aerobic and anoxic metabolism in animals. In flow calorimeters constant experimental conditions may be controlled for practically unlimited periods of time, and transitions of the environmental regime may be repeatedly performed during one experiment. The merits of a direct calorimetric flow system are demonstrated in case studies of anoxic and aerobic animal metabolism and discussed in the context of current biochemical and ecophysiological concepts.