Mass selection to improve cold and density tolerance of two maize populations, BS2 and BS3, was evaluated after three cycles of selection. During selection the populations were planted as early as possible in the spring under high plant densities at Waseca, Minnesota; Ames, Iowa; and Portageville, Missouri, which represented northern, central and southern regions of the Corn Belt, respectively. I grew the original and the three advanced cycles of BS2 and BS3 from the three selection regions at Ames, Iowa in 1978 and 1979 to evaluate for cold tolerance, and at the original selection sites and two other sites (i.e., Beaconsfield, Iowa and Sutherland, Iowa) at two dates of planting (i.e., early and normal) and two plant densities (i.e., 45.0 M plants/ha and 90.0 M plants/ha). My research was conducted: (1) to determine if mass selection using early planting dates under high plant densities was effective in improving cold and density tolerance, (2) to determine if selection changed the populations responses to dates of planting and plant densities for plant, flowering and harvest traits, (3) to evaluate changes in other traits associated with selection under these conditions, and (4) to compare results for cycles developed in the three selection regions to determine if changes due to selection in one region were expressed in other regions;Differentiation among entries for cold tolerance was effective in only one of the two evaluation environments, and, in this environment, significant improvements over cycles were shown for percentage emergence, emergence index and seedling dry weight within BS3 only. Density tolerance of BS2 and BS3 was not improved using this mass selection method. In addition, selection did not change the populations responses to dates of planting and plant densities for plant, flowering and harvest traits. Plant and ear height increases, later dates of flowering and increased grain moisture at harvest in BS2 were associated with this mass selection method. In general, changes due to selection in one region were expressed in other regions.