Electronic and vibrational relaxationsof NO in Ar, Kr, and Xe matrixes were studied using excitation spectra and time and energy resolved emission spectra. In addn. to the vibrationally relaxed emissions from a4P(v = 0), B 2P(v = 0), and A 2S+ (v = 0) in Ar and Kr matrixes, B(v = 5 and 7) emissions were obsd. in Ar matrixes. In Xe matrixes, only Rydberg A 2S+ (v = 0) fluorescence is obsd. Nonradiative Rydberg-valence transitions are obsd. in all matrixes and valence-Rydberg transitions only in Ar matrixes. The intensity ratios IA/IB/Ia are .apprx.3/9/88 in Ar, .apprx.9/3/88 in Kr, and .apprx.2-5/0/0 in Xe matrixes. The quantum efficiency for total luminescence was close to unity in Ar and Kr matrixes. The branching ratios for intramol. relaxation between Rydberg and valence states are described in terms of a model which combines the intramol. Franck-Condon factors with the spectroscopically detd. phonon Franck-Condon factors and solvent enhanced spin-orbit matrix elements. The latter increase from Ar to Xe matrixes. This increase is rationalized in terms of a semiempirical model for the heavy-atom effect on spin-orbit mixing. Weak Rydberg-Rydberg relaxation by a Dv = 2 step in Ar and a Dv = 1 step in Kr is also obsd. and interpreted in terms of a resonant Foerster-Dexter-type energy transfer. Finally in Xe matrixes, a strongly nonresonant energy transfer from the n = 1 exciton of solid Xe to the A(v = 0) Rydberg level is obsd. [on SciFinder (R)]