The very large solar proton events (SPEs) which occurred from October 19 to 27, 1989, earned substantial middle-atmospheric HOx and NOx constituent increases. Although no measurements of HOx increases were made during these SPEs, increases in NO were observed by rocket instruments which are in good agreement with calculated NO increases from our proton energy degradation code. Both the HOx and the NOx increases can cause ozone decreases; however, the HOx-induced ozone changes are relatively short-lived because HOx species have lifetimes of only hours in the middle atmosphere. Our two-dimensional model, when used to simulate effects of the longer-lived NOx, predicted lower-stratospheric polar ozone decreases of greater than 2% persisting for one and a half years past these SPEs. Previous three-dimensional model simulations of these SPEs (Jackman et al., 1993) indicated the importance of properly representing the polar vortices and warming events when accounting for the ozone decreases observed by the solar backscattered ultraviolet 2 instrument two months past these atmospheric perturbations. In an expansion of that study, we found that it was necessary to simulate the November 1, 1989, to April 2, 1990, time period and the November 1, 1986, to April 2, 1987, time period with our three-dimensional model in order to more directly compare to the stratospheric aerosol and gas experiment (SAGE) II observations of lower stratospheric NO2 and ozone changes between the end of March 1987 and 1990 at 70°N. Both the NOx increases from the October 1989 SPEs and the larger downward transport in the 1989–1990 northern winter compared to the 1986–1987 northern winter contributed to the large enhancements in NO2 in the lower stratosphere observed in the SAGE II measurements at the end of March 1990. Our three-dimensional model simulations predict smaller ozone decreases than those observed by SAGE II in the lower stratosphere near the end of March 1990, indicating that other factors, such as heterogeneous chemistry, might also be influencing the constituents of this region.