Two long term selection experiments in mice and Tribolium are used (a) to evaluate the importance of overdominant loci in the inheritance of quantitative traits and (b) to determine the number of loci responsible for the genetic variation for these traits. Mass selection is used as a tool for eliminating segregating genes that are not overdominant or pleiotropic in a negative way between fitness and the trait under selection. Intense inbreeding and selection for combining ability will be used in plateaued populations to test for residual genetic variation involving dominant genes and to determine whether there is any appreciable variation remaining that can be attributable to overdominant genes. While the questions posed relate to traits that are not direct measures of fitness the results do have relevance to the questions concerning the extent of polymorphism in nature and the kind of gene action responsible for the maintenance of genetic variation.