Measurements of NO, HNO3, and O3 concentrations and air temperature have been made as a function of latitude in the 18 to 21 km region of the stratosphere. The NO and O3 concentrations were measured in situ with chemiluminescence detectors; the HNO3 concentrations were deduced from HNO3 column densities obtained from measurements of the radiance in the 11.2 &mgr;m band of HNO3. From the NO and HNO3 measurements the sum NOy=NO+NO2+HNO3 is inferred. Comparisons with model predictions are made for the NO concentration measured as a function of latitude for spring, summer, and fall, and for the NOy mixing ratio at 20 40 ¿N. The NO measurements generally exhibit good agreement with model predictions for low and midlatitudes but poor agreement at high latitudes. The experimental NOy mixing ratios and model predictions agree within a factor of 2-1/2 or better at both 20 and 40 ¿N, and show excellent agreement in NOy latitudinal dependence.