Presumptive olfactory mucosa was excised from the heads of rat fetuses in the eleventh and twelfth days of gestation and explanted in organ cultures. At the time of explanation, the presumptive olfactory cells were recognizable by their long narrow apical processes and basally located nuclei. However at this stage they were in an early phase of differentiation as indicated by the large numbers of free ribosomes and virtual absence of microtubules and cilia in the apical cytoplasm. After three to eight days in culture, there was a progressive increase in the total number of cells in the epithelium. Differentiation in olfactory receptor cells was detectable by the appearance in the apical processes of axially oriented microtubules and centrioles or basal bodies, some of which generated cilia. At their basal ends, the cytoplasm narrowed into axons and bundles of these axons, arranged in the unique manner of olfactory nerve axons, entered the connective tissue. Olfactory receptor cells, as defined by morphological criteria, differentiated under suitable organ culture conditions in the absence of any tissue from the central nervous system.