Inbred lines generated from 10 maize population developed between 1979 and 2008 were used to investigate the inbreeding depression of tropical maize varieties developed at different breeding eras and evaluated in (optimal and stressed) condition. Across all the environments used for this study, estimates of inbred depression (I) for grain yield which ranged from 15.63% for optimum environment to 35.85 under stem borer infestation, showed differences in the severity of the effects of practicing inbreeding in each of the populations and the different environments. The highest values of inbreeding depression for grain yield were recorded under stem borer infestation. The effect of inbreeding was the most severe for var. DMR-LSR-W under borer infestation and least for DMR-LSR-Y in stress free environment. This is an indication that the responses of the maize populations to inbreeding as well as the rate of attaining homozygosity differed with environments. Across the four different environments under which the genotypes were evaluated, average inbreeding depression for grain yield were greater relative to other traits considered which should be expected since grain yield is a quantitatively inherited trait, governed by many genes each with minor effects.