The nitrate radical (NO3) is the dominant atmospheric oxidant during the night in most environments. During the day, however, NO3 has thus far been considered insignificant. Here we present daytime measurements of NO3 by Differential Optical Absorption Spectroscopy near Houston, Texas, during the Texas Air Quality Study 2000. On 3 consecutive days in August/September 2000, NO3 reached levels from ~5 ppt 3 hours before sunset to 31 ppt around sunset. Daytime NO3 had a negligible effect on the photostationary state (PSS) between O3 and NOx, with the exception of the last hour before sunset, when it significantly accelerated NO-to-NO2 conversion. On August 31, chemical reactions involving NO3 destroyed 8 (¿4) ppb Ox (= O3 + NO2) during the day and 27 (¿6) ppb at night. NO3 chemistry contributed 10 (¿7)% to the total Ox loss during the daytime, and 28% (¿18%) integrated over a 24-hour period. It therefore played an important role in the Ox budget. NO3 also contributed significantly to the daytime oxidation of hydrocarbons such as monoterpenes and phenol in Houston. The observed daytime NO3 mixing ratios can be described as a function of O3 and NOx. Above x>/3> ratios of 3%, daytime NO3 becomes independent of NOx and proportional to the square of O3. Our calculations indicate that elevated (>1 ppt) NO3 levels can be present whenever ozone mixing ratios exceed typical urban smog levels of 100 ppb.