The process of nucleolar vacuolation has been quantitatively studied in root cells of the embryo of Zea mays . During germination, we have estimated the percentage of vacuolated nucleoli, the number of vacuoles per nucleolus and the volume density of the vacuoles, the latter by application of the stereological principles of morphometry. In the dormant embryo, no vacuoles can be detected in the nucleolus. When germination occurs at 22 or 16 °C, there is a rapid increase in the percentage of vacuolated nucleoli and in the volume density (V A ), with a maximum after 4 and 8 h. At 6 °C, a temperature which does not permit emergence of the root, the percentage of vacuolated nucleoli increases regularly but never reaches the level observed at 16 and 22 °C. In parallel with the appearance of nucleolar vacuoles, the size of the nucleolus decreases statistically and shows a minimum when vacuolation is maximum. This suggests that the appearance of vacuoles within the nucleolus is the result of loss of material from this organelle. Indeed, electron micrographs show that granular components are lost from the nucleolus during the first 8 h of germination at 16 °C. According to the literature, these granular components are probably ribosomal subunits. An autoradiographic study of tritiated uridine incorporation shows that the nucleolus does not synthesize rRNA at the early beginning of germination. Hence the nucleolus is unable for several hours to rebuild the lost granular elements. Application of actinomycin D, 5-fluoro-uracil and 2-thio-uracil to the seeds, during the first 48 h of soaking, inhibits rRNA synthesis, the vacuolation process, the loss of granular components and the reduction in size of the nucleolus. These results allow us to determine some of the physiological conditions characterizing the appearance of the nucleolar vacuoles.