This experimental investigation reports the temperature-dependent reaction rate constants for electronically excited, metastable NH(a) and ND(a) radicals in the v = 0-3 levels that are transferring electronic energy to O-2 to yield O-2(b). The covered temperature range is 209-421 K. All measured Arrhenius activation energies (E-a) are essentially independent of v and fall into the range of 6.7-9.0 kJ/mol, with an average value of 7.6 kJ/mol, except for NH(a,v = 3), which has an E-a value of 3.5 +/- 1.6 kJ/mol. Obviously, for the lowest levels, vibrational excitation does not markedly accelerate the energy transfer and does not seem to be favorable to overcome the small activation barrier. From this and previous work, we propose that a vibrationally adiabatic process generates NH(X,v). In the case of NH(a,v = 3), we suggest that an additional channel opens.