ABSTRACT The role of DNA regions replicated at different times during the S period in the process of chromatin decondensation that takes place in the next G1 period, as well as in the timing of the G1 to S transition, was analyzed in synchronous populations of cells in Allium cepa L. root meristems. For this analysis, DNA bromosubstitution (10 −7 M 5-bromo-2 ′-deoxyuridine feeding) was carried out at similar times corresponding to the first, middle and last thirds of the S period prior to telophase when anoxic 313 nm irradiation was carried out. Evaluation, after Feulgen staining, of the chromatin pattern corresponding to G0 and G1 phenotypes in these synchronous cells showed that chromatin decondensation after mitosis appeared to be under the control of a double negative/positive regulatory mechanism related, respectively, to early and late replicating sequences. Thus, such decondensation was favoured when bromosubstitution took place during the first part of the S period, but delayed when bromosubstitution took place during the third part or the whole of the S period. Nevertheless, bromosubstitution at both the early and late stages of the S period delayed the timing of the G1 to S transition. In this way, the initiation of replication appears to be positively controlled by the function of early replicating DNA. Finally, the DNA sequences replicated in the middle third of the S period did not modify the decondensing capacity of the chromatin in G1 or the timing of the G1 to S transition.