The hypothesis that the antitumor prodrug temozolomide is ring-opened to MTIC which then further breaks down to a reactive diazonium ion at guanine-rich sequences in DNA has been probed by NMR spectroscopy and computational techniques. Temozolomide is stable at acid pH but decomposes to MTIC at pH > 7; in contrast, MTIC is stable at alkaline pH values but rapidly fragments in a methylating mode at pH < 7. The proximate methylating agent is the reactive methyldiazonium species. Runs of guanine residues represent an accessible nucleophilic microenvironment in DNA site-specific conversion of the prodrug temozolomide to MTIC possibly via an activated water molecule in the major groove. Molecular modeling of the structure of temozolomide indicates that the prodrug can make a favorable noncovalent encounter with DNA. The known structure-activity relationships as well as the biological and clinical properties of temozolomide can be interpreted in terms of this model.