Cephalopods are carnivores; proteins constitute their major source of energy, and ammonia is the main end‐product of amino acid catabolism. It is released as a linear function of time, by diffusion through the gill's transporting inner epithelium. A diagram of the various possibilities of ammonia transit, combining passive and active transport, is proposed (Figure 1).The metabolic costs are highest in the squids (Loligo, Illex) and lowest in Nautilus. Biotic factors, such as life cycle stage, feeding conditions and activity affect the metabolic rates, as do environmental factors such as temperature.Ammonia release is the lowest in Sepia and Nautilus, which use nitrogen gas in their shell for buoyancy. Other cephalopods are known to accumulate ammonium ions in their body for buoyancy. The relationship between buoyancy and nitrogen metabolism is placed in the context of the adaptation of cephalopods to changing life styles during the course of evolution.