The high resolution magnetic rotation spectrum of a diatomic molecule is considered. A procedure is described for calculation of the magnetic rotation signal in both first and second harmonic for an oscillating magnetic field in the low-field limit. The procedure is simpler than other methods available in the literature, and a more complete account is taken of the various possible contributions to the signal. Perturbations between electronic states that make it possible to observe magnetic rotation signals for high-J transitions are discussed, along with the associated evolution of line shape that complicates frequency measurements from magnetic rotation spectra. Examples are given from a recent magnetic rotation study of the A (3)Pi (1u)-X (1)Sigma (+)(g) system of Br-79(2).