The average diurnal variation of Na emission in the nightglow is discussed from the observations during November 1964 to April 1967 at Mt. Abu (24.6" N, 72.7" E). Mainly the Na intensity was found to vary linearly with time and on some occasions the oscillatory nature in it was also detected. In the sets of linear diurnalvariationsan anti-coveriation is found in the initial intensities of the linear variations and the associated rates of their variat,ions. This inter-relationship in the initial intensities and the rates of variations is related with the tendency for appoaching an equilibrium value in the night time sodium emission.