Nighttime to daytime variation of the mesospheric ozone abundance was measured by means of the 110.8--GHzO3 rotational line during February 1975 with a ground-based radio telescope located 31¿N, 104¿W. The daytime and nighttime frequency profiles were treated by a numerical inversion method to obtain the altitude distributions of the ozone mixing ratio. The dawn mesospheric O3 decay was observed to occur during a 50--min interval during sunrise. The early daytime reduction of O3column density is approximately 3.4¿1015 cm-2 above 60 km and is 1.0¿1016 cm-2 above 50 km, both being a reduction by a factor of about 2 over the nighttime column densities.