The first in situ measurements of turbulence in the upper Arctic mesosphere during the transition period from winter to summer were performed during the MIDAS/SPRING campaign in May 2000 from the And¿ya Rocket Range in northern Norway (69¿N). The ionization gauge CONE on board two sounding rockets identified height ranges with turbulent neutral density fluctuations which were used to determine turbulent energy dissipation rates. Accompanying in situ temperature measurements with falling spheres and remote wind measurements with a MF radar revealed the rapid seasonal change of the mesosphere's thermal structure and large scale dynamics just during the campaign. Our in situ measurements give evidence of an equally rapid change of the turbulent structure of the mesosphere within only ~10 days. Models that take into account upward propagating gravity waves which break in the mesosphere show reasonable agreement with our findings.