We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite observations of water vapor abundances, to study odd-hydrogen photochemistry in the mesosphere. The MAHRSI data sampled primarily the morning part of the diurnal variation of OH, and here we focus on one orbit of data that is representative of the MAHRSI observations during the mission. Our approach is to use a photochemical model, with input H2O abundances fixed to observed values, to simulate the diurnal variation of mesospheric HOx species. Models of OH using standard recommended HOx chemical rate coefficients are found to substantially overpredict OH between 50--65 km. Proposed modifications to HOx chemistry that produce lower OH and higher HO2 and O3, yield better agreement in the lower mesosphere but worsen the agreement at the observed OH peak near ~70 km. We find that neither standard HOx chemistry, nor models incorporating proposed HOx modifications, adequately reproduce the observed OH density profile over the entire 50--75 km altitude range for any of the observed early to mid morning local times. ¿ American Geophysical Union 1996