The odd nitrogen source strengths associated with solar proton events (SPEs), galactic cosmic rays (GCRs), and the oxidation of nitrous oxide in the Earth's middle atmosphere from 1974 through 1993 have been compared globally, at middle and lower latitudes (50¿) with a two-dimensional photochemical transport model. As discovered previously, the oxidation of nitrous oxide dominates the global odd nitrogen source, while GCRs and SPEs are significant at polar latitudes. The horizontal transport of odd nitrogen, produced by the oxidation of nitrous oxide at latitudes <50¿, was found to be the dominant source of odd nitrogen in the polar regions, with GCRs contributing substantially during the entire solar cycle. The source of odd nitrogen from SPEs was more sporadic; however, contributions during several years (mostly near solar maximum) were significant in the polar middle atmosphere. ¿ American Geophysical Union 1996