The existence of myoelastic sphincters in the terminal airways of some cetaceans is well established. Although several studies have tried to explain the functional role of these structures, none of them has been based on a comparative analysis. The present study was initiated with the purpose of investigating whether differences in the morphology of myoelastic sphincters are related to different diving patterns. We have compared serial histological lungs sections from two dolphin species with different diving patterns. In Pontoporia blainvillei, a shallow diving dolphin, myoelastic sphincters are thin rings with irregular distribution, which occasionally strangle the airway. Lagenorhynchus obscurus, with deeper dives and faster vertical displacements, presents thicker sphincters with regular distribution, generally strangling the airway completely. We conclude that these morphologies could be related to different functional demands. We suggest the existence of two groups of dolphins, one shallow diver, comprising small cetaceans with less developed sphincters whose functional activity is uncertain, and another group with a wider diving range, having strong and functional sphincters.