Abstract The baseline sister-chromatid exchanges (SCEs) and the percentage of first (M 1), second (M 2) and third or higher metaphase (M 3+) chromosomes were analysed in bone-marrow cells of male and female C57BL/6 mice and Chinese hamsters following serial intraperitoneal injections of 40 μg/g body weight (b.w.) of 5-bromo-2′-deoxyuridine (BrdUrd) and 2 μg/g b.w. of 5-fluorodeoxyuridine (FdUrd) or 40 μg/g b.w. of BrdUrd and 10 μg/g b.w. of deoxycytidine (dC). Female animals receiving BrdUrd/FdUrd showed significantly higher ( P < 0.01) baseline SCEs compared to the other groups. No sex difference in the baseline SCEs was found in animals treated with BrdUrd/dC. The distribution patterns of M 1, M 2 and M 3+ metaphases in BrdUrd/FdUrd-treated animals differ significantly from those in BrdUrd/dC-treated animals.