Autoradiography was used to localize the sites of incorporation of L-[(3)H]fucose into root tips of maize (Zea mays L. cv. S.X. 17). By light microsocpy, accumulation of label from [(3)H]fucose could be seen in the peripheral cells of the root cap. Extraction of sections prepared by freeze-substitution showed that most of the label in the cytoplasm of peripheral root-cap cells is water-soluble whereas label associated with the wall is sodiumhydroxide-soluble. In the electron microscope, glutaraldehyde-fixed peripheral cells of maize root caps are characterized by the presence of numerous dictyosomes and vesicles. The distended dictyosome cisternae and vesicles have large deposits of silver after staining with periodic acid-silver methanamine. An accumulation of material similar to that found in dictyosomes and vesicles is observed between the cell membrane and wall in glutaraldehyde-formaldehyde-fixed tissue. At the electron-microscope level label in peripheral root cap cells incubated in [(3)H]fucose for periods from 10 to 120 min was found primarily over dictyosomes and dictyosome vesicles. In pulse-chase experiments label was chased from the dictyosomes and vesicles to the exterior of the cell in 20-30 min. Less than 19% of the label in pulse-chase experiments was associated with organelles other than dictyosomes or dictyosome vesicles.