Alterations in chromosomal morphology and staining affinity were produced in mouse embryo cells afterin vivoexposure to 2000 mg/kg BrdU on day 10 of gestation. Treated females were killed either 2, 4, or 6 h after BrdU administration and embryo cells were cultured in a colcemid-containing medium for 2 h. Chromosomes exposed to BrdU for 2 h showed centromeric regions that were lightly stained by the Giemsa technique. After 4 h treatment, both centromeric and non-centromeric regions were affected with both decreased staining affinity and a loss of chromosome compactness being observed. After 6 h exposure, lighter Giemsa staining was observed throughout the chromosomes and there was an indication of chromosomal segmentation.