The olfactory epithelium of mammals is simple in appearance when fixed and stained with conventional techniques and viewed with the light microscope. In it, there are three types of cells, the olfactory receptors, the supporting, and the basal cells, all of which rest upon a thin basement membrane, demonstrable only with difficulty (1-5). In spite of its apparently simple organization and single type of receptor element, the olfactory epithelium is capable of remarkably fine sensory discriminations. Olfactory receptors respond to stimulating substances in extraordinarily small concentration. Moreover, the receptors are sensitive to an enormous range of odoriferous substances.Electrophysiological investigations (6-9) have led several investigators to suggest recently that discrimination depends upon the existence of different types of olfactory receptors with selective sensitivities to basic odors, and although the spatial distribution of receptors may account for coarse discriminations of odors, fine differentiation seems to depend upon specific sensitivity of individual receptors to particular molecular configurations (9).These findings pose the question of whether or not the olfactory receptors exhibit morphological differences corresponding to their functional heterogeneity. By way of answering this, the studies of Le Gros Clark (10-12) have demonstrated several histological differences among receptor cells. For example, the receptors vary in their affinity for silver stains, in the length and diameter of the rod processes, in the size of their terminal swellings, and in the number of olfactory hairs. They also differ markedly in their degree of degeneration after destruction of the olfactory bulb (12).It must be admitted, however, that the limited resolution of the light microscope does not permit a sufficiently detailed analysis of the olfactory receptors and their relationships to the other cells of the olfactory epithelium. For example, the receptor processes are difficult to demonstrate reliably even in the best histological preparations. In addition, many of the central processes of the * This investigation has been aided in part by Grant B-425 and RG 3784 from the United States