We have used [7-N-15]-labeled deoxyguanosine and deoxyadenosine to probe the Hoogsteen hydrogen bonding in C+. GC and T . AT DNA triplexes. The tripler used in this work consists of a 21 base pair duplex with a 15 base third strand. Use of a tripler that is formed intermolecularly, rather than one formed intramolecularly, enabled us to examine separately the single strand, the duplex, and the tripler under the same conditions of temperature and pH. The single strands and the duplexes each show only a single resonance, while the tripler samples show two resonances at each intermediate temperature, one identical to that of the duplex and another 6 to 9 ppm upfield, which we assign to the tripler. At lower temperature or lower pH the intensity of the upfield (tripler) resonance is increased, while at higher temperature or higher pH it is decreased. These results provide the first direct evidence for the Hoogsteen hydrogen bonding to the purine N7 atoms postulated for C+. GC and T . AT triplets. Moreover, the magnitude of the chemical shift changes seen for this DNA tripler suggests that N-15 NMR of appropriately labeled DNA and RNA molecules may be able to identify the presence of Hoogsteen hydrogen bonding to a purine N7 in less well defined systems. This may be of particular importance for RNA structures, where a wider variety of tertiary interactions is present.