Young male Wistar rats weighing 100 g were injected intravenously with 0.2 mg of vinblastine sulfate (VBL). At 24, 27, and 36 hours and 2, 4, and 6 days after introduction of the drug, the rats were fixed by means of perfusion and the upper incisors were removed for electron microscopy. Many secretion granules had accumulated in the young odontoblasts 24 hours after VBL administration. Newly formed predentin located between odontoblasts contained small amounts of an unusual collagen aggregate characterized by a fine, symmetrically striated structure, and paired, dense bands, which were present at both ends. After 27 hours, following the disappearance of the secretion granules in the odontoblasts, unusual collagen aggregates had increased in amount. Two days after administration, the predentin layer, which was close to the odontoblasts, was composed largely of unusual collagen aggregates. During dentinogenesis, the layer of unusual collagen aggregates gradually shifted into the dentin, where, 4 days after VBL administration, it was, to a large degree, located. Mineralization of the unusual collagen aggregates began with the deposition of fine, filamentous crystals that later grew into needle-shaped crystals 40 A thick. These findings suggest that unusual collagen aggregates are produced as a result of the secreting function of odontoblasts and are mineralized, as are normal collagen fibrils in dentin.