Seasonal and interannual variations in the intensity of internal gravity waves (IGW) at altitudes of 100--300 km are investigated using measurements of electron density, ion drift velocity parallel to the geomagnetic field, and ion temperature with the Saint Santin incoherent scatter radar between 1967 and 1987. IGW with periods of less than 6 hours produce ion temperature amplitudes of 5--80 K and ion velocity amplitudes along the geomagnetic field of 3--17 m/sec. Vertical profiles of IGW intensity have maxima in the altitude regions 140--180 and 250--280 km and vary on annual and interannual timescales. Usually, the IGW intensity is at its maximum in winter and minimum in summer. Strong interannual variations are also observed, whose long-term trends sometimes appear to follow the solar cycle, either positively or negatively, but are often irregular. Model calculations show that maxima of IGW intensity during solar minimum can be explained by the diminution of atmospheric density and ion drag, which leads to increased amplitudes of IGW propagating to the thermosphere from lower layers of the atmosphere. ¿ American Geophysical Union 1994