Sodium nightglow measuremnts made at S¿o Jos¿ dos Campos (23.2¿S) with a tilting filter photometer over a period of 2 years show that on the average the intensities reach a minimum close to local midnight. Between midnight and dawn the intensity remains fairly constant or increases slightly. The most consistent feature is the premidnight decrease in intensity. Seasonal variations show peak intensities at the equinoxes, the stronger one occurring in autumn. In a few cases the nocturnal variation does not follow the average trend, showing rather strong peaks in intensity which have been related to dynamic effects. Calculations based on the photochemistry described require the reaction rate for Na+O3→NaO*+O2 to be much larger than the rate for the analogous hydrogen reaction, in agreement with a recent suggestion in the literature that the above reaction proceeds through the so-called electron jump mechanism.