The paper aims to assess the impact of migration and natural reproduction of population to the changes in ethnic structure of population by the case study of three multi-ethnic functional urban regions in Slovakia. All three regions are located at the Slovak-Hungarian ethnic border and are characterized by larger proportion of Slovaks and lower proportion of Hungarians in the core than in the ring. The regions were chosen also with intent to provide a sample with various processes of spatial redistribution of population taking place, which are described in the paper. The ethnic structure at the levels of whole regions, their basic components - cores and rings - is introduced briefly. The analyses of changes in the ethnic structure and its relationships with the natural reproduction and migration of population provides interesting results. It was proved that the migration is a driving force in changes of ethnic structure in analysed regions, even though the patterns of natural reproduction behaviors between ethnic Slovaks and Hungarians are traditionally different. Since the most intensive process of spatial redistribution of population is the decentralization from the cores to the rings of the regions driven by migration, the ethnic structure of the rings tends to approximate to that in the cores.