Magnetic circular dichroism (MCD) and absorption spectra are reported for the A (3)Pi(i) <-- X (3)Sigma(-) transition of imidogen (NH) isolated in Kr, Xe, and N-2 matrices at cryogenic temperatures (similar to1.4-20 K) and over a range of magnetic field strengths (0-5 T). The results are analyzed by the method of moments, and parameters are extracted by fitting the experimental data to a model in which the A (3)Pi(i) term is split by spin-orbit (SO) coupling interactions and the A (3)Pi(i) term is split by spin-spin and higher-order SO coupling. The analysis indicates that the ground-state NH radicals behave essentially as free rotors in the noble-gas matrices but as hindered rotors in solid N-2. Trends in excited-state SO coupling constants are attributed to the external heavy-atom effect.