Binding of the bis-intercalators echinomycin and N,N'-di(9-acridinyl)spermidine or the mono-intercalators 9-aminoacridine and ethidium to DNA induces hypersensitivity of adenines towards reaction with diethyl pyrocarbonate. It is proposed that this hyperreactivity is due to the DNA helix unwinding and extension induced by intercalation, thereby exposing N7 in the major groove, and not as previously suggested to the formation of Hoogsteen base pairing. Hypersensitivity of thymines towards oxidation with permanganate is also induced upon binding of these drugs (especially the bis-intercalators) to DNA. This thymine hyperreactivity is both sequence- and intercalator-dependent, thereby indicating the potential of KMnO4 as a useful probe for analyzing the structure of intercalator-DNA complexes in solution.