Full-dimensional quantum dynamics of the A-state photodissociation of NH(3) and ND(3) is characterized on potential energy surfaces recently developed by Li et al. [Theor. Chem. Acc. 118, 9 (2007)]. The A<--X absorption spectra of the two isotopomers were obtained for several vibrational levels on the ground electronic state. The calculated positions and lifetimes of low-lying umbrella resonances on the excited A electronic state are found to reproduce experimental trends and shed light on the multidimensional dynamics. The agreement with measured spectra is reasonable, but assignment of some individual features remains incomplete.