Metal-mediated base pairs represent a powerful tool for the site-specific functionalization of nucleic acids with metal ions. The development of applications of the metal-modified nucleic acids will depend on the availability of structural information on these double helices. We present here the NMR solution structure of a self-complementary DNA oligonucleotide with three consecutive imidazole nucleotides in its centre. In the absence of transition-metal ions, a hairpin structure is adopted with the artificial nucleotides forming the loop. In the presence of Ag(i) ions, a duplex comprising three imidazole-Ag(+)-imidazole base pairs is formed. Direct proof for the formation of metal-mediated base pairs was obtained from ¹J(¹⁵N,¹⁰⁷/¹⁰⁹Ag) couplings upon incorporation of ¹⁵N-labelled imidazole. The duplex adopts a B-type conformation with only minor deviations in the region of the artificial bases. This work represents the first structural characterization of a metal-modified nucleic acid with a continuous stretch of metal-mediated base pairs.