A new technique was developed to measure the amount of 5-bromo-2 '-deoxyurdine (BrdUrd), a thymidine analogue, rapidly and conveniently in blood, and it was applied to measure the concentration of BrdUrd during the initiation of a paradigm for the labeling of DNA to measure the induction of sister chromatid exchanges (SCE) by genotoxic agents. Radio labelled BrdUrd was used and blood was drawn from the abdominal aorta in pregnant Sprague-Dawley derived D rats. Using the hplc, the BrdUrd peak was easily identifiable and separable from its metabolites. The BrdUrd level in the blood stabilized in 30 minutes, and a level of about .4 mg% was enough to label the bone marrow and fetal tissue for SCE. Also, the time course of the BrdUrd metabolites suggest a constant BrdUrd metabolism by the organism. Using absorbance alone, blood levels of BrdUrd during paradigms of biological significance can be measured.

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