The primary DNA lesion induced by malondialdehyde, a byproduct of lipid peroxidation and prostaglandin synthesis, is 3-(2′-deoxy-β-d-erythro-pentofuranosyl)-pyrimido[1,2-a]purin-10(3H)-one (M₁G). When placed opposite cytosine (underlined) at neutral pH in either the d(GGTMTCCG)⋅d(CGGACACC) or d(ATCGCMCGGCATG)⋅ d(CATGCCGCGCGAT) duplexes, M₁G spontaneously and quantitatively converts to the ring-opened derivative N²-(3-oxo-1-propenyl)-dG. Ring-opening is reversible on thermal denaturation. Ring-opening does not occur at neutral pH in single-stranded oligodeoxynucleotides or when T is placed opposite to M₁G in a duplex. The presence of a complementary cytosine is not required to stabilize N²-(3-oxo-1-propenyl)-dG in duplex DNA at neutral pH. When N²-(3-oxo-1-propenyl)-dG is placed opposite to thymine in a duplex, it does not revert to M₁G. A mechanism for the conversion of M₁G to N²-(3-oxo-1-propenyl)-dG is proposed in which the exocyclic amino group of the complementary cytosine attacks the C8 position of the M₁G exocyclic ring and facilitates ring opening via formation of a transient Schiff base. Addition of water to the Schiff base regenerates the catalytic cytosine and generates N²-(3-oxo-1-propenyl)-dG. These results document the ability of duplex DNA to catalyze the transformation of one adduct into another, which may have important consequences for mutagenesis and repair.