A written intelligence test with verbal, reasoning, and spatial subscales was administered to two comparable national samples of 1,000 thirteen-year olds in Sweden, tested in 1961 and in 1966. The increases were greater for girls than for boys, and the changes occurred simultaneously with several changes in social and educational conditions. To analyze the relations between test score changes and changes in the environment, the samples were divided into ecologically more homogeneous subgroups by geographic location and population density, and test scores were correlated with ecological indices. Test scores were positively related to a global ecological index (population density), more so for boys than girls, and more so at the first than the second occasion. The latter finding indicated that an equalization between different areas had taken place during the 5-year interval. Changes for boys were positively related to environmental changes in the direction of urbanization, while changes for girls were more related to changes in the educational system stressing equality of treatment of both sexes. This impact was most striking in the spatial visualization test. Statistical data are included. (Author/CP)