The spectral-intensity distributions of the resonance line of sodium from the high-pressure sodium plasma generated in a transparent sapphire envelope have been measured under various operating pressures (50–200 torr) in the presence of the foreign gases (Xe, Ar, He, and Hg). The resonance line was strongly self-reversed and slightly asymmetrical-broadening to red was observed when mercury was added. The main broadening mechanism was resonance broadening. Van der Waals broadening by foreign perturbers, especially by mercury, contributes to the slightly asymmetrical broadening. The electron temperature of the plasma (about 3200°K on the axis of the sodium-xenon plasma, and about 4100°K on the axis of sodium-mercury-xenon plasma) is measured by Bartels' method. However, the temperature obtained from the absolute intensity of the resonance line, that is the temperature describing the relative population density of the upper level (3 p) of the resonance line to the ground level (3 s) of that line, is higher than the electron temperature measured by Bartels' method by about 600°K. The mean free path of the resonance photon is comparable to the tube radius in both lateral peaks of the self-reversed resonance line. Therefore, the population density of the upper level of the resonance line at a particular point of the plasma is determined not only by the local value of the electron temperature and density but also by self-absorption of resonance photons emitted from other parts of the plasma, that is to say, the L.T.E. approximation is invalid in the resonance line.