One of the inherent difficulties associated with the accurate detection of sister chromatid exchanges (SCE’s) in vivo is the maintenance of a continuous supply of 5-bromo-2’-deoxyuridine (BrdU) for differential staining of sister chromatids. We have developed a simple, reproducible intraperitoneal technique which has the advantages of continuous BrdU infusion and is less stressful than presently available methods. Additionally, a gentle trypsinization methodology was developed to obtain relatively large numbers of metaphases from fetal tissues. Using these techniques, we examined the induction of SCE by 7,12-dimethylbenz(a)anthracene (DMBA) when injected into pregnant CD rats on days 11, 13, 15, and 16 of gestation. The levels of SCE induced were slightly higher in fetal than maternal tissue, although not significan