Atomic nitrogen number densities in the upper thermosphere measured by the open source neutral mass spectrometer (OSS) on Atmosphere Explorer-C during 1974 and part of 1975 have been used to construct a global empirical model at an altitude of 375 km based on a spherical harmonic expansion. The most evident features of the model are large diurnal and seasonal variations of atomic nitrogen and only a moderate and latitude-dependent density increase during periods of geomagnetic activity. The model also suggests the presnece of a substantial semiannual variation and of nighttime minima at mid-latitudes. Maximum and minimum N number densities at 375 km for periods of low solar activity are 3.6¿106 cm3 at 1500 LST (local solar time) and low latitude in the summer hemisphere and 1.5¿106 cm3 at 0200 LST at mid-latitudes in the winter hemisphere. The amplitude of the diurnal variation at equatorial latitudes is about a factor of 8.