The development of the olfactory and vomeronasal epithelia of the mouse has been investigated from the 9th day of gestation until shortly after birth. On the 10th day axons emerge from the base of the olfactory epithelium to reach the olfactory bulb primordia on the 11th day, at which time the olfactory dendrites first appear. On the 13th day a distinction between the elongate nuclei of the embryonic stem cells and the rounded nuclei of the differentiating receptors is visible. A basal layer of stem cells remains mitotically active from the 13th day of gestation onwards, and after the 15th day the majority of mitoses occur in this layer. It is suggested that from the 15th day of gestation onwards the nuclei situated most apically become those of the supporting cells. The glands of Bowman are first visible on the 17th day of gestation. The diverticulum of the vomeronasal organ begins to form on the 11th day and the development of its sensory epithelium resembles that of the olfactory organ except for the absence of basally situated stem cell nuclei in the later stages of the vomeronasal organ.