Ultraviolet photolysis of highly vibrationally excited NH3((X) over tilde (1)A(1)(')) in the 5nu(N-H) band has been studied using a crossed laser and molecular beams method. Relative cross sections of N-H bond dissociation via the (A) over tilde <--(X) over tilde transition have been determined by the measurement of the action spectrum, utilizing a (2+1) resonance enhanced multiphoton ionization scheme of product H atoms. The obtained cross section for the photolysis of the 4nu(1)+nu(3) state (4 quanta in the symmetric stretching and 1 quantum in the antisymmetric stretching) was 1.23+/-0.06 times as large as that of the 5nu(1) state. To interpret the ratio of the obtained cross sections, we have evaluated the Franck-Condon factors for these vibrational states, by utilizing time-dependent wave-packet calculations on the potential energy surfaces constructed by an ab initio molecular orbital procedure. (C) 2003 American Institute of Physics.