During prenatal development the respiratory epithelium surface of the rat's nasal septum underwent the following changes. At intra-uterine day E14 there was a transformation from State I, cells with primary cilia only, to cells which also had microvilli (Stage II). Anterior parts of the nasal septum retained microvilli throughout further development. Posteriorly, centriole multiplication (Stage III) and formation of shafts of secondary cilia (Stage IV) occurred from around E16 and E18 onwards, respectively. From E18 to E20 numbers of cells with cilia increased at an overall rate of about 6 X 10(6) cells/cm2/day. Respiratory cilia and microvilli grew, on average, by about 0.3 micron/day and 0.1 micron/day, respectively. At Stage V, beginning around E19, the cilia became aligned within cells and, at Stage VI, beginning around E21, became synchronised between cells. Respiratory ciliogenesis in the nose is most precocious near the olfactory epithelium. The formation of respiratory cilia starts after that of olfactory cilia. However, unlike olfactory epithelium surfaces those of ciliated respiratory epithelia resembled those of adults around birth.