The v' = 0-4 vibrational levels of the b4S- state of NO in Ar and Kr matrtixes were selectively excited in the near-IR by a tunable laser from the metastable a4P state previously populated by an ArF excimer laser. One of the decay channels of the b state is a radiationless transition to low vibrational levels of the B2P state. By monitoring the time- and energy-resolved B2P->X2P fluorescence the energies were detd. of the b, v' levels and the propensity rules of the b, v' -> B, v" transitions. The results are in good agreement with the previously proposed model assuming an important role for the b4.sum.- state in the radiationless relaxation from the A2S+ (Rydberg) to the final radiative valence states B2P and a4P. [on SciFinder (R)]