The temperature data from the High Resolution Doppler Imager on board the Upper Atmospheric Research Satellite are used to study mesospheric dynamics. Data spread over 7 years (1992--1998) in the height region of 75--105 km are analyzed to study the seasonal and long-term changes in the mesopause height and temperatures. During December solstice (1992), the mesopause is at the high level of 100 km in the Northern Hemisphere with a temperature of 190 K, and around 40¿S, the mesopause changes to a low level of 88 km with a cool temperature (150 K) of 40 K less than the high-level mesopause. In June solstice (1993), the mirror image of this pattern is observed. This kind of two-level mesopause structure is consistently seen during all solstices at the same height and the transition occurring at the same latitudes. The high-level mesopause with temperatures of 180--190 K and the low-level mesopause with temperatures of 140--150 K are observed. The two-level mesopause is not observed in equinoxes, and a single-level mesopause is seen around 100-km altitude in both hemispheres. The annually averaged temperature data show 5 K warming of high-level mesopause during 1992 and 1993, which may be due to Pinatubo volcanic aerosols. This warming is comparable but less than the 9 K warming at 100-km height observed by She et al. <1998> at 41¿N. The time series analysis of temperature at 99 km shows a decrease of 1.2 ¿ 0.15 K/yr over the equatorial region during 1992--1998. The lowest mesopause temperatures at northern high latitudes are observed between May and August, which is consistent with the period of polar mesosphere summer echo observed by using VHF radar at these latitudes.