The degree of synchrony in the course of the interphase periods G₁, S and G₂ and in the initiation of mitosis in the several nuclei of each cell of a polynucleate population induced by treatment with 0.1% caffeine, in root meristems of Allium cepa, through inhibition of cytokinesis in two successive cell divisions is analysed by means of labelling with ³H-thymidine.—The S period is initiated simultaneously in all the nuclei of each polynucleate cell, which supports the hypothesis of a factor present in the cytoplasm that is responsible for inducing DNA synthesis.—However, all the nuclei in a polynucleate cell do not pass from the S period to the G₂ period simultaneously, those surrounded by the greatest amount of cytoplasm, generally the outer nuclei, being the first to complete the S period (early nuclei) and beginning the prophase before their fellow-nuclei in the same cell (late nuclei).—From the metaphase onwards, however, all the nuclei in a polynucleate cell continue to develop synchronously. The synchronizing mechanism has a twofold aspect: the shortening of the G₂ period in the late nuclei and the lengthening of it in the early ones and, on the other hand, an arrest of prophase in the early nuclei until the late ones have caught up, which suggests the existence of an inhibiting factor produced by the late nuclei capable of acting upon the early ones through the cytoplasm.