We present an analysis of odd nitrogen species measured by the CRISTA experiment during the Space Shuttle mission STS 66 in November 1994. Observations of several nitrogen species are compared with modeled fields obtained from three-dimensional simulations performed with a chemical transport model (CTM). A sequential trace gas assimilation system is used in order to constrain the simulations by CRISTA observations of long-lived species, and total amounts of stratospheric nitrogen (NOy) derived from measurements of individual nitrogen components (NO2, N2O5, HNO3, and ClONO2). Comparisons of measured and modeled NOy species focus on significant diurnal variations and intense longitudinal variations observed by CRISTA. ¿ 2000 American Geophysical Union