The techniques of light and electron microscopy were used to study the sequence of changes in the fine structure of the developing nonciliated bronchiolar cells in the last quarter of gestation in the mouse, and up to the 3rd postnatal day. Control animals were compared with two groups that received either dexamethasone or ambroxol. At the beginning of the study no identifiable Clara cells were seen in any group, and phenotypically differentiated cells were seen until 1 day post partum in every group examined. Dexamethasone appears to increase the glycogen deposition found in the cells, reaching its maximum 1 day prior to delivery. Ambroxol induced an earlier development of the secretory organelles, beginning on day 17 of gestation. In the differentiated cells more secretory dense granules were found in this group; and at the 3rd day post partum a great accumulation of secretory product that filled large areas of cytoplasm was seen only in the ambroxol-treated group. The significance of these finding is discussed.