In Raphanus sativus, the mitotic chromosomes are quite short and, on reaching the cell poles, soon undergo extensive unravelling. By late telophase and early interphase, only a few chromosome segments, believed to correspond to the centromeric regions, are still visible in the form of chromocentres closely associated with the nuclear envelope. Although interphase nuclei show little internal structural differentiation, high-resolution radioautography has permitted us to establish which of them have reached the early, mid and late S periods. In early S nuclei, only the nucleolus and the euchromatin which pervades the nuclear cavity become labelled. By the mid S-period, the diffuse chromatin and nucleolus incorporate less thymidine and DNA synthesis is initiated within the peripheral chromocentres. Subsequently, the radioautographic grains become restricted to the chromocentres. The finding that certain late S nuclei exhibit loosely organized chromocentres strongly suggests that these heterochromatic chromosome segments undergo important conformational modifications during DNA replication. Finally, the presence of radioautographic grains over the lacunar regions of the nucleolus in early and mid S nuclei demonstrates that intranucleolar DNA replicates during the earlier portion of the S-period.