Continuous measurements of the vertical distribution of atmospheric sodium, made over a number of complete diurnal cycles, show the existence of strong semidiurnal oscillations in total abundance and height. The amplitude of the abundance variation, about 15% of the mean, is about twice that predicted for the 2,2 mode of the semidiurnal tide, and its phase, with maxima at 0400 and 1400 LT, is in good agreement with tidal theory. The vertical oscillation, with an amplitude of 4 km at a height of 100 km, is about 3 times the expected amplitude, and the measured vertical wavelength of 38 km is in good agreement with theory, although the phase is not. A strong diurnal oscillation, observed only at heights below 82 km, is interpreted as being the result of photochemical reactions between sodium and other photochemical reactions between sodium and other atmospheric constituents. The lack of any appreciable 24-hour component in the total abudance variation implies either a residence time for total sodium of more than 3 days, or source and sink functions whose diurnal variations, unless identical, are very small.