Marine mammals rely on their diving capacity to survive as this determines their foraging efficiency. Ultimately, their anatomy and physiological capacity, and the limitations imposed by the environment determine the maximal time underwater. Thus, the behavioral decisions undertaken by marine mammals would be related with the different combination of these factors. As air breathing mammals, respiratory function may alter the diving capacities and limit foraging. Further, respiratory disease has been reported as one of the major causes of mortality in these species and may have consequences for their respiratory function and diving capacities. In addition, the ability of marine mammals to efficiently use the available O2 stores during diving has been recognized as an important component that determines diving behavior and the duration of the breath-hold. However, the increasing human activity in the marine ecosystem (e.g., climate change, overfishing, contamination, etc.) is resulting in environmental alterations that could force marine mammals to perform dives beyond their physiological scope. In addition, these perturbations may disrupt the normal physiological mechanisms that enhance diving, or may also increase their susceptibility to disease, which could have consequences on their diving capacity. Therefore, a better understanding of species-specific normal respiratory function and capacity could help improve our understanding about respiratory limitations and the functional consequences of respiratory disease, and how these affect the diving capabilities of marine mammals. In addition, studies evaluating the metabolic rate, or O2 consumption rate, during diving in different species will help understand the energetic requirements of underwater behaviors, such as foraging or travelling. This information could help understand interspecific limitations that drive behavioral decisions within a changing environment. Such information would be vital to enhance conservation efforts of those populations suffering increased direct or indirect impacts that could affect their future survival. The present thesis is aimed to provide basic information about respiratory function and energetic requirements during in-water activities in one of the species facing an increased habitat loss: the Pacific walrus (Odobenus rosmarus divergens). In addition, the present investigation assessed the use of spirometry as a non-invasive diagnostic tool to evaluate respiratory health and the functional consequences of respiratory disease in the bottlenose dolphin (Tursiops truncatus). These objectives were performed with the participation of trained animals housed in professional care and were developed in three independent chapters briefly summarized below: In Chapter I, basic respiratory function was measured in three adult Pacific walruses. The results showed an enhanced ventilatory capacity (e.g., higher tidal volume and lung compliance, and high respiratory flows maintained over the entire respiratory manoeuvre) compared with terrestrial mammals. Respiratory function was assessed in different body positions on land and in water, which showed flow limitations while lying on land and increased respiratory flows when in water. These results provide basic information to understand respiratory limitations in this species, and suggest that respiratory function in semi-aquatic marine mammals should be evaluated both on land and in water. In Chapter II, lung function testing (spirometry) was used as a non-invasive method to assess respiratory health in three adult bottlenose dolphins. Lung health was assessed through the development of lung function indices adapted for the respiratory capacity of dolphins, and the evaluation of the flow-volume relationship. The results were compared with clinical diagnostics (e.g., blood and sputum samples) and chest radiographs, and showed that lung function testing could detect respiratory disease, evaluate functional changes, and assess treatment efficacy in dolphins trained to exhale maximally while in water. In addition, the results indicated that spirometry could provide diagnostic information in stranded individuals while breathing spontaneously. Thus, the results presented in this chapter showed that lung function testing is a potential non-invasive method to evaluate respiratory health in trained and stranded dolphins. In Chapter III, the O2 consumption rate was measured in three adult Pacific walruses while floating at the water surface, and during short and shallow stationary dives and subsurface swimming. The results are similar to previously results reported for adult pinnipeds where measured metabolic rates during inactive periods (on land or in water) were greater than those expected from similarly sized terrestrial mammals. In addition, metabolic rate during diving was lower as compared with periods at the surface, as previously reported for other pinniped species. This shows that the walrus has behavioural or physiological means to limit metabolic costs during diving. The data presented in this chapter could help improve previous bioenergetic models aimed to quantify the consequences of environmental change in the Pacific walrus.