Enamel formation was reviewed by morphology and radioautography in rat incisors. Labeled amino acids and sugars were used as matrix precursors whereas labeled calcium monitored mineral deposition. All ameloblasts synthesize organic material, but only cells in the zone of secretion release labeled matrix. The pattern of matrix deposition indicates that enamel rods are elaborated by Tomes' processes within cavities formed by interrod partitions. The latter are elaborated by cytoplasmic projections from adjacent ameloblasts. Initially-labeled matrix is added as a band near the cells. With time the label randomizes throughout the entire immature enamel and most of it is lost in the zone of maturation. However, a glycoprotein component attributed to remnants of Tomes' process membrane persists in mature enamel. Labeled calcium is incorporated into crystals which grow at a uniform rate throughout the entire layer of enamel in the zone of secretion and up to the middle of the zone of maturation. The ribbon-like crystals are built close to the cell membrane and elongate as the cell recedes. Crystal elongation occurs in the same location as new matrix is deposited; that is, rod crystals are related to Tomes' processes and interrod crystals, to cytoplasmic projections. The crystals grow to full size mainly by thickening and this growth presumably displaces the organic matrix.