Natural dives are usually short and aerobic, involving swimming effort that is not energetically costly. Present evidence indicates few, if any, major biochemical enzyme adaptations that would promote an extended anaerobic capacity in marine mammals. We have discussed the lack of evidence for unusual biochemical adaptations for anaerobiosis and the importance and characteristics of natural aerobic dives. What explanation can we offer for the known capability of some marine mammals, such as Weddell seals and sperm whales, to remain submerged for over an hour? Attributes that serve them well for shorter dives are important also for long dives, such as: (a) a large total oxygen store relative to body size, and (b) parsimonious use of blood oxygen due to the lowered energy requirements of various organs. During long dives, some organs deplete their oxygen stores, become predominately dependent on anaerobic metabolism, and build up large quantities of lactate. The result is a degree of fatigue that leaves the animal incapable of further intense diving activities for some time. Consequently, long anaerobic dives are quite rare in nature.