A high resolution study of magnetic-field-induced second harmonic generation in sodium vapour was performed using a single-frequency continuous-wave dye laser. Resonant enhancement was obtained by tuning the laser to the frequency of the 3S-40 two-photon transition of the sodium atoms. Coherent parametric emission of the second harmonic radiation ocurred in the presence of a symmetry-breaking magnetic field by means of a coherent electric-quadrupole emission at twice the laser frequency. A theoretical model of the second harmonic generation (SHG) was developed, and includes the roles played by atomic selection rules, Zeeman splitting and eigen function mixing in a magnetic field, and the effects of homogeneous and inhomogeneous broadening. The use of a single-frequency laser and an atomic nonlinear medium allowed an experimental investigation of a SHG process in unprecedented detail. This provided astringent test of the theoretical model developed to predict the properties of the nonlinear interaction in terms of fundamental atomic parameters. The excellent agreement obtained between theoretical and experimental results vindicated the theoretical methods used in the modelling.