Nucleolar vacuoles in eight regions of the root apex ofZea mayshave been studied. The volume of the vacuoles ranges from about 1 μm3to 0.001 μm3with a mode value between 0.075 and 0.01 μm3. The number of vacuoles within a nucleolus ranges from 0–20 and is correlated with nucleolar volume. In all regions studied the average volume of the nucleolus occupied by vacuoles is about 3 percent. It is suggested that the metabolic capacity of the nucleolus may be reflected in the rate of formation of the vacuoles and the discharge of their contents to the cell.