The neutral atmosphere compositon experiment on Atmosphere Explorer C measured N2, O, Ar, and He densities during a magnetic storm in February 1974 at altitudes down to about 160 km. At latitudes above 45¿N, N2, and Ar densities generally increase during the storm, while He and O densities decrease. Below 45¿N all densities tend to increase during the storm. The density increases at perigee indicate that density or temperature profile changes are taking place below 160 km. The return to prestorm conditions is very slow, demonstrating the integrating effect of the atmospheric response. A recent theoretical model incorparting thermospheric circulation and diffusion effects reproduces the logitudinally averaged data including latitude trends and the asymmetry about the storm maximum. Comparison with the mass spectrometer and incoherent scatter empirical model shows qualitative agreement with latitude trends but not with storm asymmetry, while the earlier J71 model based on total mass density is not in agreement with observed latitudinal trends. No significant correlation is found with the short-term variations of the ap index. At any fixed altitude and for latitudes above 45¿N (perigee) the density variations are closely correlated with invariant (or magnetic) latitude, although invariant latitude alone is not adequate to order the data completely. A close correlation is found between in situ O/N2 measurements and in situ and ground-based ionosonde measurements of electron density.