Regional and temporal variations in the ABO and Rh blood group gene frequencies were studied in the rural population of Lithuania. The total material, comprising blood group data for about 37,000 individuals, was subdivided according to place of residence into 401 parishes, 44 administrative districts and 6 ethnolinguistic regions. A significant regional heterogeneity was found. Gene diversity figures were higher than those observed in some other countries when studied at similar hierarchical levels. The gene frequency variation at the district level was compared to expected effects of genetic drift and migration and it was estimated that a migration rate of 5-10% would counter genetic drift in the rural population of Lithuania.