Two changes in cytoplasmic granules, corresponding to colloid droplets and dense bodies, in rat thyroid follicular cells during 24 hr were analyzed morphometrically in the apical, intermediate, and basal parts of the cells. The volume and surface densities of the cytoplasmic granules and their volumes and surface areas in the apical and intermediate parts varied bimodally over a 24-hr day, being high at 1200 and 0000 hr and low at other times. In apical and intermediate regions, colloid droplets were predominant at 1200 and 0000 hr, whereas the cytoplasmic granules became heterogeneously or homogeneously electron-dense at other times. In the basal part, these morphometric parameters were not synchronized with those in the other two regions, showing lower values. The basal cytoplasmic granules were heterogeneously or homogeneously electron-dense. Small, homogeneously dense granules appeared in both the apical and intermediate parts and in the basal part with a certain time lag. These granules were often in contact with colloid droplets in the lumenal two parts at 1200 and 0000 hr. Their numerical densities were low in the apical part, but high in the intermediate and basal parts. These results suggest that newly formed colloid droplets migrate from the apical through the intermediate to the basal part, changing in appearance as they go. Moreover, it seems likely that small, homogeneously dense granules are a final form of cytoplasmic granule. They may be reused for degradation of colloidal protein.