We have studied normal thyroid specimens from 20 euthyroid patients with the scanning electron microscope. A significant negative correlation was found between the diameter of the apical cell surface and the density of microvilli. Large follicles were usually surrounded by smooth-surfaced cells with a large apical diameter, while the smaller follicles were surrounded by smaller cells with numerous and large microvilli. Exceptions to this rule were, however, not uncommon, and the morphological differences between neighbouring cells in one and the same follicle were as great as those between the cells of different follicles. Concurrent transmission electron microscopy showed that the morphology of the apical cell membrane reflects the activity of the follicular cells. The scanning electron microscopical findings thus indicate that the thyroid may have an intrafollicular functional cycle as important or more important than an interfollicular cycle. Our study shows that one to five cilia are regular constituents of the normal follicular cells and that their number is not correlated with the diameter of the apical cell surface nor with the density of microvilli. Although we do not rule out the possibility that they may be fetal rudiments, we suggest that they may play a functional role probably by stirring the colloid.