When the perigee of the Atmosphere Explorer C satellite reached the nothern latitudes (68¿N) by mid-April 1974, descending and ascending portions of the orbits stretched symmetrically across the northern hemisphere. Corresponding local solar times at altitudes above 400 km were 1700 hours and 0200 hours, respectively, thus providing the opportunity to study the diurnal variation of the neutral constituents. Atomic nitrogen densities, measured with the open source neutral mass spectrometer, show a pronounced diurnal variation. At 400 km the density ratios derived from measurements of the descending and ascending portions are 5.2 for N, 4.9 for N2, and 2.0 for O. Absolute densities of atomic nitrogen agree within a factor of 2 or 3 with densities derived from neutral-ion reactions. Measurements taken close to the geomagnetic pole show a decrease in atomic nitrogen densities when an increase in molecular nitrogen is observed.