Shallow-diving, coastal bottlenose dolphins (Tursiops truncatus) and deep-diving, pelagic pygmy and dwarf sperm whales (Kogia breviceps and K. sima) will experience vastly different ambient pressures at depth, which will influence the volume of air within their lungs and potentially the degree of thoracic collapse they experience. This study tested the hypotheses that lung size will be reduced and/or thoracic mobility will be enhanced in deeper divers. Lung mass (T. truncatus, n = 106; kogiids, n = 18) and lung volume (T. truncatus, n = 5; kogiids, n = 4), relative to total body mass, were compared. One T. truncatus and one K. sima were cross-sectioned to calculate lung, thoracic vasculature, and other organ volumes. Excised thoraxes (T. truncatus, n = 3; kogiids, n = 4) were mechanically manipulated to compare changes in thoracic cavity shape and volume. Kogiid lungs were half the mass and one-fifth the volume of those of similarly sized T. truncatus. The lungs occupied only 15% of the total thoracic cavity volume in K. sima and 37% in T. truncatus. The kogiid and dolphin thoraxes underwent similar changes in shape and volume, although the width of the thoracic inlet was relatively constrained in kogiids. A broader phylogenetic comparison demonstrated that the ratio of lung mass to total body mass in kogiids, physeterids, and ziphiids was similar to that of terrestrial mammals, while delphinids and phocoenids possessed relatively large lungs. Thus, small lung size in deep-diving odontocetes may be a plesiomorphic character. The relatively large lung size of delphinids and phocoenids appears to be a derived condition that may permit the lung to function as a site of respiratory gas exchange throughout a dive in these rapid breathing, short-duration, shallow divers. J. Morphol., 2010. © 2010 Wiley-Liss, Inc.