Protein synthesis was studied in dental germs of new-born cats by quantitative electron microscopical autoradiography, after an intravenous injection of tritiated proline. The poor labeling of the external epithelial layer, of the stellate reticulum and the stratum intermedium stood in strong contrast with the high labeling of the ameloblast and its process. Five minutes after the intravenous injection of tritiated proline, a high radioactivity of the Golgi complex was noted. It decreased progressively 30 min, 1 hr and 24 hr after the intravenous injection. The majority of the proteins diffused towards the ameloblastic process and the extra-cellular space through ameloblastic bodies, elaborated in the Golgi apparatus. The ameloblastic endoplasmic reticulum presented a relatively moderate labeling. The mineralizing enamel was diffusely labeled after 24 hr and silver grains were even located in the peripheral layers of dentine.