The third largest solar proton event in the past thirty years took place during July 14–16, 2000, and had a significant impact on the earth's atmosphere. These energetic protons produced both HO_x (H, OH, HO₂) and NO_x (N, NO, NO₂) constituents in the mesosphere and upper stratosphere at polar latitudes (> 60° geomagnetic) of both hemispheres. The temporal evolution of increases in NO and NO₂ during the event at northern polar latitudes were measured by the UARS HALOE instrument. Increases in mesospheric NO_x of over 50 ppbv were found in the HALOE measurements. Measurements from the UARS HALOE and NOAA 14 SBUV/2 instruments indicate short‐term (∼day) middle mesospheric ozone decreases of over 70% caused by short‐lived HO_x during the event with a longer‐term (several days) upper stratospheric ozone depletion of up to 9% caused by longer‐lived NO_x. We believe this is the first time that the three constituents NO, NO₂, and ozone were all measured simultaneously during a proton event. The observations constitute a dramatic confirmation of the impact of a large particle event in the control of ozone in the polar middle atmosphere and offer the opportunity to test theories of constituent changes driven by particle precipitation.