A simple silver staining technique for use at the electron microscopic level, consisting only of treatment with aqueous silver nitrate at high temperature for a prolonged time, was applied to thin sections of root tip meristems of Vicia faba. This technique contrasted the fibrillar component and the granular component in interphase nucleoli as a reflection of the degree of packing. In contrast, silver impregnation was scarcely discerned in chromosomes. A comparison of silver staining and conventional double staining showed that the fibrillar centres did not always respond positively to silver. During the course from metaphase to late anaphase the nucleolus organizing secondary constriction was always seen as a heavily impregnated region and the electron density of the cytoplasm increased, probably due to dispersed nucleolar material. An argyrophilic substance began to accumulate on chromosomes in late anaphase. In the beginning of telophase a uniformly impregnated nucleolus was formed at the secondary constriction. It is concluded from these results that argyrophilic substance is associated with RNA-containing structures rather than DNA-containing structures. The silver staining technique presented here is very convenient and favourable, especially for plant cells, to detect specifically the nucleolus organizing region and to survey the nucleolar material during mitosis at the electron microscopic level.