We have studied the dependent of sister-chromatid exchange (SCE) production on the presence of oxygen during incubation of cells in 5-bromodeoxyuridine (BrdUrd) by analyzing the SCE yields in second-division chromosomes of Allium cepa L. meristem cells. We considered the cases between 10 and 100% oxygen in the incubation medium. Under these experimental situations no significant modifications in cell-cycle duration were obtained. Our results lead us to conclude that: (1) in oxygenated treatments, cells incubated during 2 consecutive cycles in the presence of BrdUrd show significantly higher SEC frequencies (in second-division chromosomes) than those treated only during the first cycle; (2) SCE frequencies increase as a function oxygen tension; and (3) oxygen-dependent SCE frequencies increase as a function of oxygen tension and (3) oxygen-dependent SCes appear to be found by exchanging post-replicative DNA portions. These observations suggest that the experimental protocol reported here could be useful in determining both the precise moment of action of an agent during the cell cycle and the DNA portions actively involved in physically and chemically provoked SCEs.