The present study consists of three related experiments which are concerned with the development of national attitudes in children between the ages of seven and twelve. It was predicted on the basis of a structural interpretation of Allport's three-stage developmental theory of prejudice that national attitudes will increase at first due to increasing consistency of judgment and decrease afterwards due to cognitive differentiation. The hypothesis is partially confirmed with respect to the attitudes of children towards other countries but not confirmed with respect to the attitudes of children towards people who are perceived as foreigners. A tentative explanation is offered for the last finding. It was shown moreover that the attitudes of older children display more cognitive balance than those of younger children. In connection with the last problem, a quantified modification Bf Heider's theory of balanced states was introduced.