The nonciliated bronchiolar epithelial (Clara) cells are principally distributed in the distal airway, and characterized by the presence of abundant agranular endoplasmic reticulum and electron-dense granules. In the present study, changes in the glycogen, intracellular organella and cytoskeleton occurred in perinatal rat Clara cells were studied by H. E. staining, PAS staining, transmission electron microscopy and detergent perfusion method. The cytoplasm of Clara cells in the prenatal rat contained glycogen and was intensely positive on PAS staining. In 1 to 3-day-old rat, Clara cells with abundant mitochondria, smooth-surfaced endoplasmic reticulum, and filaments in supranuclear area appeared among glycogen-rich Clara cells. In 4 to 7-day-old rat, almost all Clara cells showed no PAS reaction, increased electron-dense granules concomitant with the increase of rough-surfaced endoplasmic reticulum. Abundant filaments and microtubules were observed in the supranuclear area. These results suggest that Clara cells mature after birth and filament formation in these cells occurs with cell maturation and function.