The influence of an Ar matrix on the electronic splitting energies of the a1D state of the NH radical and on the radiative transition rates of the b1S+ -> a1D electronic transition is investigated employing two different models for the nearest-neighbor Ar atoms and taking intramol. electron correlation of the radical into account. One of the models uses nonpolarizable Ar atoms kept frozen at their SCF description, the other one takes their polarizability into account within the framework of a generalized self-consistent reaction field theory, for which the first calcns. employing correlated wave functions are reported here. These models are checked with regard to their ability to reproduce other ab initio data. The calcd. splitting energies, combined with the results of calcns. on an embedded NH/Arn cluster using Ar-Ar and state-dependent ab initio Ar-NH pair potentials, allow the assignment of trapping sites of NH in the matrix. The detd. transition rates give some insight into the mechanism of matrix-induced enhancement of a dipole-forbidden electronic transition.