The mass spectrometer experiment on the Explorer XVII satellite has yielded new data on the concentrations of the major components of the neutral upper atmosphere over a two month period in the spring of 1963. These data compare favourably with results from the total density experiments on the same spacecraft as well as satellite drag data and direct measurements made from rockets. The night measurements are in general consistent with an isothermal atmosphere at a temperature of 650–700°K while the daytime scale heights indicate a temperature of 825 ± 75°K. There was a large variability in the number densities at a given altitude, indicating a strong sensitivity to changes in energy inputs to the atmosphere, particularly to changes in magnetic activity. Average quiet daytime conditions near 260 km altitude were observed to be: N2,1.5 ± 0.5 × 108 cm−3; O, 2.4 ± 0.4 × 108 cm−3; He, 1.4 ± 0.3 × 106 cm−3. Average quiet nighttime conditions at 400km were: N2, 2.0 ± 1.0 × 106 cm−3; O, 1.0 ± 0.4 × 107cm−3; He, 6.0 ± 2.0 × W cm−3. The large horizontal change in the location of the satellite during a measurement precludes the interpretation of the data in terms of a simple vertical profile, but does oner the possibility of investigating local time or other horizontal gradient effects on the atmospheric constituents.