Abstract With the aim of finding out factors involved in chromosomal condensation, the kinetics of both metaphase accumulation and the subsequent formation of restitution nuclei in the presence of 1 mM colchicine have been analysed in Allium cepa L. meristems. Restitution nuclei are formed by decondensation of the chromosomes from the c‐mitoses and the reformation of nuclear envelopes around them. The study has been carried out in control conditions as well as in conditions which either inhibit protein synthesis (1 μg/ml cycloheximide) or modify the accuracy of transcription (by near ultraviolet light irradiation under anoxia of cells with bromosubstituted‐DNA).The results suggest that the high condensation of chromosomes observed in colchicine‐arrrested metaphases depends on sustained synthesis of protein, and that the DNA regions coding for such proteins are transcribed not before late prophase.By using cells, the genome of which has been partially bromosubstituted, fed bromodeoxyuridine during different fractions of this S period, it could be inferred that the coding regions for the proteins involved in chromosome condensation are probably multiple since they replicate throughout the whole S period.