Simultaneous measurements of NO3, along with those of O3, NO2, and aerosol extinction coefficient, have been performed during the night by the AMON instrument since 1992 at high and midlatitudes and by the SALOMON instrument since 1998 at midlatitude. Observations are conducted using the stellar and lunar occultation methods, respectively. Vertical profiles of NO3 are obtained after inversion of the optical depth spectra recorded from 650 to 670 nm, including the 662-nm absorption line. Five profiles at midlatitude and two profiles at high latitude are presented. Comparisons with a box model constrained with measured ozone and temperatures (and NO2 at high latitude) have been performed, taking into account the uncertainties in the rate constants of the reactions leading to NO3 equilibrium. The modeling results can reproduce part of the observations, taking into account possible errors in the rate constants, temperature, or NO3 absorption cross sections. Some disagreements nevertheless remain between observations and modeling outputs. In the middle stratosphere they could result from gradients of temperature. Below 30 km, other phenomena could be invoked to explain the disagreements. At high latitude the presence of solid polar aerosols induces an artifact in the data reduction. At midlatitude, large increases observed in the NO3 concentration profiles obtained between 1992 and 1994 are real. A speculative hypothesis involving volcanic aerosols is proposed. ¿ 2001 American Geophysical Union