Human olfactory neuroepithelium and respiratory mucous membrane in fetal and adult whole-mount sections were studied. In the fetus, the olfactory neuroepithelium extends from the roof of the nasal cavity to the midportion of the nasal septum and onto the superior turbinate in a continuous fashion. In the adult, the zonal distribution of supporting, sensory receptor, and basal cells is frequently disrupted, and the supporting and sensory receptor cells are often depleted or degenerate. The degree of the degeneration of the adult olfactory neuroepithelium varies from case to case. The most striking feature in the adult is the replacement of large areas of olfactory neuroepithelium with respiratory epithelium. The extensive replacement of olfactory neuroepithelium with respiratory epithelium points out the sampling problem related to small, random biopsy specimens of the olfactory area.