To study the influence of 5-aminouracil (5-AU) on the cell cycle of meristematic root tip tissue the following quantities, among others, have been investigated during and after the 5-AU treatment: mitotic index, labeling index, rate of DNA synthesis, and alteration of the frequency of cells with nuclei of certain DNA content.The rate of DNA synthesis was determined by measuring the rate of TdR-3H incorporation, after a 5-AU/TdR antagonism could be excluded.Following from these investigations, 5-AU reduces the rate of DNA synthesis; it does not block it completely, as 5-fluoro-UdR does.The reduction of the rate of DNA synthesis prolongs the S phase, which results in an accumulation of cells in this state, in fact favorably in the last third of the S phase.In this connection the possibility of a favored reduction of DNA synthesis in heterochromatic areas is discussed.Although 5-AU also influences G2, no accumulation of cells in this state has been observedOn the basis of the present results, the assumption made by several authors that 5-AU acts in the same way as 5-fluoro-UdR seems invalid.