The mesospheric nightglow from the hydroxyl (OH) radical has been characterized using measurements taken by the Improved Stratospheric and Mesospheric Sounder (ISAMS) instrument on the UARS satellite. In this paper, the variations with latitude of the emission integrated vertically over the OH layer, for both solstice and equinox conditions, are analyzed and compared to other measurements and with model predictions. A semiannual variation with a minimum at solstice is found at low latitudes. An annual variation is found at high latitudes with a springtime minimum, which appears earlier than predicted by the models. Zonal maps of the OH emission show longitudinal structures with a timescale of days that suggest the effect of planetary waves on the emission process. ¿ 2001 American Geophysical Union