To extend our investigation of the anatomy of sensory systems in highly adapted aquatic and terrestrial mammals, we have analyzed the distribution of a particular population of efferent neurons in the cetacean and human primary auditory cortex using an antibody to non-phosphorylated neurofilament protein (SMI32). The neurofilament protein triplet is differentially distributed within neuronal subpopulations in the primate and cetacean neocortex. In primates, it appears that the somatodendritic domain of a subset of pyramidal neurons furnishing specific corticocortical connections contains high concentrations of neurofilament protein. In the human primary auditory cortex these neurons are located in layers III, V and VI, whereas in cetaceans they are concentrated almost exclusively in the cortical efferent layer IIIc/V. Previous analyses have shown that SMI32 immunoreactivity in the cetacean neocortex is uniformly distributed among functionally different areas, while in human neocortex, the distribution of SMI32-positive neurons exhibit a high degree of regional and laminar specialization that is correlated with the functional and anatomical diversity of the cortical areas. In addition, the overall distribution of SMI32-immunoreactive neurons in the cetacean neocortex is comparable to that observed in paralimbic areas of the human, suggesting that the cetacean neocortex has retained many features of phylogenetically older cortical regions.