Rat liver DNA was radioactively labelled by administration of [3H]thymidine following partial hepatectomy. Two weeks later, the rats were treated with the carcinogen, dimethylnitrosamine. DNA was isolated and fractionated by elution from benzoylated DEAE-cellulose with NaCl and caffeine solution. The caffeine-eluted fraction was increased by administration of dimethylnitrosamine. This increase was proportional to the dose of carcinogen injected and persisted for at least 24 h after administration of the carcinogen. These data, together with the results of hydroxyapatite chromatography, suggest that the DNA contains short single-stranded sections associated with much longer regions of native DNA.