The ultrafast dynamics are reported of bubble formation following excitation of the A 2S+ 3ss(v = 0) Rydberg state of NO trapped in Ar matrixes. The study is carried out by a fs pump-probe technique. The detected signal is the depletion of Rydberg fluorescence induced by Rydberg-Rydberg transitions. Possibly the bubble is formed on a timescale of a few ps. [on SciFinder (R)]