The Atmosphere Explorer-C satellite carries a neutral mass spectrometer having a quasi-open ion source which can be operated either in a normal mode or in one which utilizes the velocity of the vehicle (~8.5 km/s near perigee) to distinguish between incoming ambient particles and ambient particles which have struck instrument surfaces and become accommodated. In the normal mode it performs essentially like a closed source instrument. The combined use of the two modes makes possible the determination of ambient densities of chemically active constituents such as atomic oxygen, as well as inactive species. An analysis of over 80 orbits between early February and late June, 1974, when the spacecraft's perigee was in the northern hemisphere, gave average particle densities at 200 km and mid-latitudes as follows: n (N2), 3.0¿109 cm-3, n (O2), 2.4¿108 cm-3; n(O), 2.5¿109 cm-3. Comparisons are made with other experiments. The n (N2) and n (O2) values agree well with those obtained from rocket-borne mass spectrometers. The n (O) concentration is approximately one half of the value given in the Jacchia (1971) model. There is no evidence for an increase in average n (O2) from early February to late June; this result disagrees with reports of large seasonal variations.