The effect of 2 model chemical mutagens on DNA was evaluated with the alkaline single cell gel electrophoresis (SCG) (Comet) assay in 5 mouse organs--liver, lung, kidney, spleen and bone marrow. Mice were sacrificed 3 and 24 h after the administration of the direct mutagen ethyl nitrosourea (ENU) or the liver-targeting promutagen p-dimethylaminoazobenzene (DAB). Each organ was minced, suspended at a concentration of 1 g/ml in chilled homogenizing buffer (pH 7.5) containing 0.075 M NaCl and 0.024 M Na2EDTA, homogenized gently using a Potter-type homogenizer at 500-800 rpm set in ice, and then centrifuged nuclei were used for the alkaline SCG assay. ENU induced DNA damage in cells all of the organs studied DAB, on the other hand, produced a positive response in the liver only. We suggest that it may be possible to use the alkaline SCG assay using a homogenization technique to detect the genotoxicity of chemicals in vivo in their target organs.