In the last 6 years a series of 22 sounding rockets were launched in order to investigate the dynamic state of the mesosphere and lower thermosphere. Of these flights, 19 were performed at high latitudes, either from the And¿ya Rocket Range (17 flights) in northern Norway (69 ¿N) or from Esrange (2 flights) in northern Sweden (68 ¿N). An ionization gauge mounted on board these sounding rockets measured neutral density fluctuations down to very small spatial scales of a few meters. During several flights, small-scale density fluctuations were found in layers of a few kilometers thickness. Subsequent analysis of these fluctuations indicates that they were caused by turbulent motions. The high resolution of these measurements makes it possible to unambiguously deduce turbulent energy dissipation rates &egr; from the spectra of the relative density fluctuations. The &egr; profiles and the corresponding heating rates obtained at high latitudes show a significant and systematic seasonal variation: Whereas in winter the turbulent heating rates are comparatively small (typically 0.1 K/d and 1--2 K/d below and above ~75 km, respectively) in the entire mesosphere and lower thermosphere, much stronger mean values of ~10--20 K/d are observed around the summer mesopause (~90 km). During none of the seven summer flights have we detected any noticeable turbulence in the middle and lower mesosphere. Turbulence is confined to a relatively small height region of 78--97 km during summer but covers the entire mesosphere from 60 to 100 km during winter. From our measurements we arrive at the curious conclusion that turbulent heating in the mesosphere is strongest at the coldest part of the atmosphere, namely, at the polar mesopause in summer. Our observations imply that turbulent heating is an important contribution to the energy budget of the upper mesosphere in summer, whereas it is presumably negligible in the entire mesosphere in winter. Mean turbulent velocities wturb and mean turbulent diffusion coefficients K do not exhibit such a distinct seasonal variation. In the lower and upper mesosphere, typical values for wturb are 0.3 and 1--3 m/s, respectively, and typical values for K are 4 and 100 m2/s, respectively. The seasonal variation of mean profiles of turbulent parameters as obtained by high-resolution in situ techniques puts a serious constraint on models dealing with the energy budget of the upper atmosphere, in particular the parameterization of subgrid process in terms of mean state quantities, e.g., used in gravity wave breaking scenarios.¿ 1997 American Geophysical Union