By use of an analytic potential energy surface developed in this work for nitric acid, the quasi-classical trajectory method was used to simulate intramolecular vibrational energy redistribution (IVR). A method was developed for monitoring the average vibrational energy in the OH (or OD) mode that uses the meansquare displacement of the bond length calculated during the trajectories. This method is effective for both rotating and nonrotating molecules. The calculated IVR time constant for HONO2 decreases exponentially with increasing excitation energy, is almost independent of rotational temperature, and is in excellent agreement with the experimental determination (Bingemann, D.; Gorman, M. P.; King, A. M.; Crim, F. F. J. Chem. Phys. 1997, 107, 661). In DONO2, the IVR time constants show more complicated behavior with increasing excitation energy, apparently due to 2:1 Fermi-resonance coupling with lower frequency modes. This effect should be measurable in experiments.