Mesospheric water vapor profiles have been derived from ground-based microwave measurements at the 22.235-GHz H2O spectral line. The measurements were made in the atmospheric thermal emission mode in spring 1984 at Pennsylvania State University (41¿N, 78¿W). The spectral data obtained were integrated and averaged over three different time scales: the total observational period, the monthly period, and the 12-hour (day/night) period. The Phillips-Twomey constrained inversion technique was modified for and then used in this study to improve the retrieval of H2O profiles. The inversion results of the total period averaged spectra suggest a mesospheric H2O mixing ratio decreasing with altitude. Values found were from 4 to 5 parts per million by volume (ppmv) at 65 km to less than 1 ppmv at 80 km, which is in basic agreement with most other observations reported. In addition, the monthly H2O retrievals showed a tendency of increasing H2O content toward the latter part of spring. This is consistent with the predictions from recent dynamic-chemical models that include the effects of gravity wave breaking. On the other hand, the results of the study of the daily variability of mesospheric H2O support the presence of planetary wave activity in the middle mesosphere. ¿ American Geophysical Union 1988