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 HOx(H,OH,HO2) and NOx(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 NOx 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 HOx during the event with a longer-term (several days) upper stratospheric ozone depletion of up to 9% caused by longer-lived NOx. 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. ¿ 2001 American Geophysical Union