AbstractTopically applied tetradecane is known to produce marked hyperplasia of sebaceous glands in the inner side skin of male rabbit pinnas. An autoradiographic and ultrastructural study was performed on these sebaceous glands to determine the changes in the proliferation activity of the peripheral sebaceous cells and their morphological changes.In a microautoradiographic study using3H‐thymidine, the labeling index of the peripheral cells decreased at an early stage and thereafter rapidly increased up to ten times the normal level. The ultrastructural study of the treated pinnas showed a remarkable increase in the number of undifferentiated sebaceous cells. They were oval to cuboid in shape and formed several cell layers. Among these cells, electron dense sebaceous cells existed; this morphological change was already evident at hour (hr) 24 and gradually became remarkable with the time course.From these findings, it is clear that topically applied tetradecane may cause temporal damage to the peripheral sebaceous cells in the early stages and then active proliferation of these cells, probably by a stimulation of their DNA synthesis, results in production of a large number of sebaceous cell