The ultrastructure of embryonic root cells of Zea mays was studied from the quiescent stage (dry seed) to 72 h of germination. Semithin and ultrathin sections of tissues fixed with only glutaraldehyde and embedded in Epon were observed after usual section staining and after cytochemical reactions specific for DNA or preferential for ribonucleoproteiNS. In quiescent cells, dense chromatin forms a network which fills a great part of the nucleoplasm. Following germination, gradual dispersion of chromatin occurs: total dispersion is reached at 24 h. After 48 h the chromatin appears moderately condensed again. The nucleolus is compact and predominately fibrillar in dry cells. At 48 h a typical pars granulosa is differentiated. At 8 h a pronounced vacuolation of the nucleolus is observed; nucleolar vacuoles persist until 72 h but become less numerous. During the first 8 h of germination a nucleolus organizer region (NOR) in an eccentric position is associated with the nucleolus; by 24 h and later this NOR has disappeared. No DNA can be visualized in the nucleolar matrix between 0 and 8 h of germination, whereas later, when the nucleolus is reactivated. DNA is always detected in the nucleolar matrix and vacuoles. During the first 72 h of germination, heavily contrasted areas, rich in ribonucleoproteins and appearing to be fibrillar texture, are found in the nucleoplasm, often in closed contact with the dense chromatin. In quiescent cells dense ribonucleoprotein granules, approximately 40 nm in diameter are found dispersed or clustered in the nucleoplasm; after 8 h larger (50 nm), dense ribonucleoprotein granules are found frequently clustered in granular areas in the extranucleolar space. At 8 h of germination, when the nucleolus is temporarily highly vacuolated, unusual 35-nm ribonucleoprotein granules are found both in the smallest vacuoles and on the periphery of the nucleolus.