We describe a three-dimensional quantum mechanical study within the nonreactive infinite order sudden approximation (IOSA) for the title systems. The study was performed using a recently introduced global potential energy surface (Bradley et al. J. Chem. Phys. 1995, 102, 6696). Integral total cross sections for the two reactions, namely, NO + NH and NO + ND, were calculated as a function of kinetic energy in the range 0.05-0.50 eV. Using these cross sections, temperature-dependent rate constants were calculated. Our main findings are: (a) The energy-dependent cross sections for the two isotopic reactions are very similar; still, the cross sections for NO + NH are larger at the low-energy region while those for NO + ND are somewhat larger at the high-energy region. (b) The two cross section curves start to increase at zero kinetic energy, indicating the absence of a potential energy barrier or the existence of a mild one at most. (c) Compared with quasi-classical-trajectory cross sections the present quantum mechanical cross sections for NH + NO are half as large at the low energy region but tend to become similar at the higher one. (d) The calculated rate constants for NH + NO were compared with experiment (for a wide range of temperatures) and with one quasi-classical trajectory result (at T 300 K). There is a very encouraging agreement at the high-temperature region (1200 less than or equal to T less than or equal to 5000 K) but large discrepancies (1 order of magnitude difference) at low temperatures. The fit with the (single) classical result was reasonably good. (e) Rate constants calculated for ND + NO were found to be very similar to the rate constants of NH + NO. A single measured value, k(T 300 K) (2.7 +/- 0.4) 10(-11) molec(-1) cm(3) s(-1), reported here for the first time, is equal to half the corresponding value for NH + NO, namely, k(T = 300 K) (5.5 +/- 0.3) x 10(-11) molec(-1) cm(3) s(-1), and it therefore fit the QM curve slightly better.