This study explores the feasibility of identifying long term changes in mesospheric water vapor as a result of increasing level of methane in the atmosphere and the solar cycle variation of Lyman &agr;. The study is based on recent measurements of water vapor in the mesosphere and the solar Lyman &agr; flux from the UARS (Upper Atmosphere Research Satellite) HALOE (Halogen Occultation Experiment) and the SOLSTICE (Solar Stellar Iradiance Comparison Experiment) instruments during the declining phase of the solar cycle 22. The solar activity during this period decreased from a near maximum to a near minimum level. The analysis of these data sets, in conjunction with the NASA/GSFC two dimensional chemistry and transport model suggests that on a seasonal time scale, the temporal changes in mesospheric water vapor are largely controlled by the vertical advection associated with the meridional circulation. On the time scale of a solar cycle, H2O may vary by about 30--40% near the mesopause height (~80 km) to about 1--2% in the lower mesosphere (60--65 km) caused by the solar cycle modulation of Lyman &agr;. In comparison, the secular increase in H2O related to methane increase in the atmosphere is about 0.4%/year at all heights in the mesosphere.¿ 1997 American Geophysical Union