We propose to develop an in vivo system which would enable the examination of sister chromatid exchanges (SCE) in cells of the colonic epithelium of rodents. Methodology needs to be worked out so that dividing cells from deep within the Crypts of Lieberkuhn can be obtained in sufficient numbers. 5-Bromodeoxyuridine (BrdU-dye techniques will be used to visualize both baseline and drug-induced SCE.