The response of mesospheric constituents to energetic electron deposition events is modeled on a diurnal scale for two precipitation scenarios. In the first example, upper limit electron fluxes representative of diffuse aurora are employed. The other case considers the 10-day precipitation event of May 1992 observed by the Upper Atmosphere Research satellite Particle Environment Monitor (UARS/PEM). Since the UARS Halogen Occultation Experiment (UARS/HALOE) water vapor measurements indicate large variability from day to day as well as at different longitudes on the same day, water is used as a parameter in studying the diurnal response of different species to particle precipitation. Water amount affects the diurnal variation of ozone by changing the concentration of hydrogen radicals, creating a water-dependent temporal response of the depletion of ozone by precipitating ionizing energetic electrons and protons. For constant particle fluxes, ozone depletion at a given altitude varies throughout the day at variance with model results where the Sun is assumed to have an average solar zenith angle throughout the day. At night, ozone depletion remains nearly constant and different from daylight values. Nighttime and twilight OH and HO2 are significantly enhanced during electron precipitation, Since precipitation often occurs between L=3 and L=6, where L is the distance in earth radii from the center of the Earth dipole to the equatorial crossing of the field line in question, as far south as 35 ¿N in the North American longitude zone, particle precipitation is one of the factors determining OH and HO2 concentrations within this latitude range, particularly when the magnetic index Ap is enhanced. Because of the large diurnal changes associated with the amount of particle-related ozone depletion, fixed microwave sites measuring ozone and water will be better able to determine the atmospheric effects of particle precipitation than satellite measurements of ozone. Diurnally averaged models are inadequate for predicting the detailed ozone response to charged particle precipitation. ¿ 1999 American Geophysical Union