Cytochemical studies were undertaken to analyze the effect of kinetin on quantitative and qualitative changes in histones and endogenous RNA content in differentiating metaxylem cells ofZea maysroots. Microspectrophotometric analysis revealed close association between increase in the DNA: histone ratio and increase in RNA content of differentiating metaxylem cells of both, control and kinetin treated root apices. The kinetin treatment caused a larger increase in RNA content per cell but the increase in the DNA: histone ratio did not differ significantly from the developmentally correlated increase observed in the controls. However, the kinetin induced amplification of RNA is associated with an elevation of the arginine: lysine ratio of the histones. It is suggested that kinetin may bring about removal or acetylation of the lysine rich fraction and that this event is a factor in the elevation of the transcriptional activity of chromatin.