In the scanning electron microscope (SEM), the structural organization of rat incisor enamel was divided into five regions: the initial prismless layer at the dentin-enamel junction; the decussating rods of inner enamel; intermediate enamel, where the structure changes from inner to outer enamel; outer enamel with parallel rods; and the surface prismless layer. There were alterations in the orientation of the ameloblasts and characteristic changes in the morphology of the Tomes' processes during the secretion of each of these regions. Initially, the developing Tomes' processes were seen within shallow pits which also contained fibrous material from dentin matrix. During the deposition of inner enamel, alternate rows of Tomes' processes were held at right angles to each other by cytoplasmic projections which originated from the proximal regions of the Tomes' processes. The cytoplasmic projections reorganized to perpendicular orientation relative to the enamel surface during the synthesis of outer enamel. Tomes' processes of outer enamel appeared as elongated flaps extending parallel to the long axis of the tooth. In the late secretory stage, the microvillous projections disappeared and the Tomes' processes were withdrawn into button-shaped structures which coalesced to form a smooth surface that rested on the enamel.