A rocket-borne mass spectrometer has been developed for high-time-resolution and high-accuracy measurements of the number densities of nitrogen, n(N2), and argon, n(Ar), in the altitude region from 90 to 130 km. Primary aims of the experiments are the determination of altitude profiles for the ratio n(Ar)/(n(N2) and for small-scale fluctuations of n(N2). From the profiles we deduce the strength of eddy mixing, and from the fluctuations we deduce properties of instantaneous turbulence. We report on the results of four successful experiments, all performed during wintertime at polar latitudes. Near 100-km altitude, eddy coefficients Ke were found to range from 102 to >103 m2/s, the spectral index &xgr; of the power spectral density ranged from -1.2 to -2.0, and the rates of dissipation of turbulent energy, ϵ, ranged from 2 to 30 mW/kg. ¿ American Geophysical Union 1990