Two periods of extremely large solar proton events (SPEs) occurred in the past 30 years, which forced significant long-term polar stratospheric changes. The August 2--10, 1972, and October 19--27, 1989, SPEs happened in stratospheres that were quite different chemically. The stratospheric chlorine levels were relatively small in 1972 (~1.2 ppbv) and were fairly substantial in 1989 (~3 ppbv). Although these SPEs produced both HOx and NOy constituents in the mesosphere and stratosphere, only the NOy constituents had lifetimes long enough to affect ozone for several months to years past the events. Our recently improved two-dimensional chemistry and transport atmospheric model was used to compute the effects of these gigantic SPEs in a changing stratosphere. Significant upper stratospheric ozone depletions >10% are computed to last for a few months past these SPEs. The long-lived SPE-produced NOy constituents were transported to lower levels during winter after these huge SPEs and caused impacts in the middle and lower stratosphere. During periods of high halogen loading, these impacts resulted in interference with the chlorine and bromine loss cycles for ozone destruction. This interference actually led to a predicted total ozone increase that was especially notable in the time period 1992--1994, a few years after the October 1989 SPE. The chemical state of the atmosphere, including the stratospheric sulfate aerosol density, substantially affected the predicted stratospheric influence of these extremely large SPEs. ¿ 2000 American Geophysical Union