Cross sections for photoabsorption of N H3, N H2 D, NH D2, and N D3 near 298 K were measured in the spectral range of 110-144 nm using radiation from a synchrotron. Absorption cross sections and oscillator strengths of N H3 agree satisfactorily with previous reports; those of N D3 are improved over those in a previous report, whereas those of N H2 D and NH D2 are new. The oscillator strengths of transitions to D, D′, D″, F, and G states are nearly the same among all four isotopic variants, but those to D and E states vary substantially. Observed absorption bands are arranged into vibrational progressions in accord with known Rydberg transitions. All progressions show a common trend of vibrational intervals increasing with vibrational quantum numbers. The Rydberg orbitals for states D (3d e″), D′ (4s a1′), D″ (3d a1′), D (4p e′), and E (4d e″) are readily assigned with quantum defects determined in these experiments, but assignments for F (5d e″) and G (6d e″) are uncertain. Absorption cross sections of dissociative continua underneath discrete structures are larger for N H2 D and NH D2 than for N H3 and N D3, indicating that the rate of dissociation of ammonia might increase when its symmetry is broken.