A study has been made of the influence of polyuridylic acid (poly U)-adenosine (A) interactions upon the NMR proton relaxation behavior of H2O protons in aqueous solutions doped with Mn++ ions. Factors investigated included the effect of adenine oligonucleotide chain length, nucleoside concentration, and temperature upon the relaxation behavior of H2O. The results show that in all cases where solution and temperature conditions favor 2 poly U:A triple strand formation, a significant enhancement in H2O relaxation rate occurs over that for poly U or nucleotide solutions alone. Moreover, plots of relaxation enhancement versus temperature show well-defined transition regions with the enhancement approaching values for free poly U above the transition temperature, TM. The data also show that TM increases with increasing adenosine concentration and oligomer chain length. No comparable changes were observed for poly U solutions containing guanosine or cytidine derivatives. The increased relaxation rate in 2 poly U:A solutions is consistent with a more rigid secondary structure in the complex than in poly U; melting of this structure leads to an increase in polymer segmental mobility and a corresponding decrease in relaxation rate. The "melting" transitions and variation of TM with concentration and chain length have been interpreted quantitatively in terms of recent statistical models and yield values of delta H = -20 plus or minus 3 kcal/mol (base triplet) and delta S = -63 plus or minus 10 cal/mole degrees K (base triplet) in satisfactory agreement with results of optical studies.