A treatment with the analogue of thymine 5 Aminouracil (5-AU) can induce, some time after its removal, a good mitotic synchronization in root meristems of higher plants. The mechanism of action of 5-AU is still unknown. According to some Authors 5-AU may act by inducing an accumulation of the cells in some specific point of the cell cycle (the S phase in general, the end of the S phase, or G2). Since we showed in a previous work that nuclei in the last part of the S phase replicating heterochromatic DNA can be cytologically identified on the basis of their peculiar labelling, we used this feature as a tool to elucidate the stage of the S phase in which the cells grown in 5-AU appear to be blocked. According to our results, this point should be in the last 1/3 of the S phase, just before the onset of heterochromatic DNA replication.