The UARS Microwave Limb Sounder obtained measurements of ClO, HNO3, and O3 inside the Arctic lower stratospheric vortex during two intervals in February and March 2000. The data show evidence of significant chemical processing in February, consistent with the exceptionally cold conditions that prevailed earlier in the winter. Ozone at 465 K decreased by 0.04¿0.01 ppmv/day in early February, implying chemical loss rates greater than those observed during the 1995--1996 winter, which was also unusually cold in the lower stratosphere. A persistent depression in the HNO3 abundances in late March, well after polar stratospheric cloud activity had ceased, suggests a moderate degree (~20%) of denitrification around the 465 K level at 70¿N and higher equivalent latitudes. This is the strongest evidence yet seen for the occurrence of denitrification in the Arctic over spatial scales large enough to be detected in satellite measurements. ¿ 2000 American Geophysical Union