The paper presents and discusses a large set of in situ measurements of fine-scale structure in the mesosphere and lower thermosphere at high latitudes. The experimental technique used is a rocket-borne electrostatic positive ion probe with linear amplification and high precision and time resolution. This type of probe has proved to be very efficient. The accuracy and limitations of the experiment are discussed. The measured ion density fluctuations are used as passive tracers for motions in the nonionized air. Quantitative estimates of the characteristics of turbulence have been derived. The method of analysis is carefully discussed and related to previous work, and a revision of earlier methods is proposed. The results show a characteristic minimum in turbulent intensity at or below the mesopause region. ¿ American Geophysical Union 1990