Young Wistar male rats were injected intravenously with 2 mg/kg of vinblastine sulfate. After three, six, 12, 24 and 48 hours, the upper incisors were dissected out of the animals, and their young odontoblasts were observed in an electron microscope. Three to six hours after the injection, the regular polarity of the nuclei and the cytoplasmic organelles were dramatically changed following a decrease in the number of microtubules. Disturbance in protein synthesis was suggested at this stage by unusual shapes and the arrangement of the rough endoplasmic reticulum and loss of the polysomal arrangement of the ribosomes attached to the reticulum. After more than 12 hours, accumulation of secretion granules and the occurrence of bodies, containing a few strands of collagen-like fibrils, were frequently seen in various portions of the cells, indicating retarded export of the secretion substance. Occurrence of a number of unusually short fibrils in predentin was another prominent feature noticed in later stages. The fibrils were about 3000A long by 2000A or 600A wide, and many of the 2000A wide fibrils were characterized by a banded structure which resembled the previously reported SLS collagen. The 600A wide fibrils were usually banded with about 600A periodicity.