Comparisons between the composition measurements of the San Marco 3 magnetic mass spectrometer and the altitude-extrapolated Ogo 6 composition model have been made. At altitudes of 22, 250, and 280 km, where local time variations are significant, agreement between the measurements and model are within ¿30% for total oxygen content (0+202) and mass density. Molecular nitrogen density comparisons are within ¿40% at 220- and 250-km altitude. There are smaller systematic variations of ¿15% with local time for these constituents, probably resulting from daily variation amplitude and phase changes with altitude which are not adequately accounted for in the diffusive equilirbium extrapolation of the model to lower altitudes. There is poor agreement for helium in the daily variation mainly due to the type of diffusive equilibrium model used to represent the Ogo 6 data which cannot propagate the required change of phase with altitude; ratios of model to observed densities are a factor of 2 or more at certain local times. Comparisons of densities in the range from 137- to 225-km altitude indicate agreement within ¿40% for molecular nitrogen, while comparisons of total oxygen content and mass density agree within about ¿30%. Below about 170-km altitude the apparent altitude variation of the helium ratios implies a strong departure of the observed densities from those predicted by the diffusive equilibrium model.