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This study was carried out to estimate the genetic parameters for grain weight of two maize populations, for verifying their genetic potential under contrasting conditions concerning water availability. Forty nine half-sib families (HSF) were randomly taken from each of the two populations, UFVM 100 and UFVM 200, in order to evaluate the tolerance to water deficit, using two sets of experiments representing water deficit and optimum irrigation conditions. The experiments were set up under a 7 x 7 lattice design, with two replications. Sprinkler irrigation was applied and a tensiometer used for its management. The experiments were irrigated whenever the soil water potential showed -0.03 MPa. However, the experiments under water stress were not irrigated during seven days, until the soil water potential reaches -0.09 to -0.1 Mpa, at vegetative, flowering and grain filling stages. The experimental plot comprised a row length 5m, 0.90m row spacing and plants spaced 0.20m within rows. Grain yield was determined for each plot. Analysis of variance was performed, involving the 49 HSF of each population, in each production system, and corresponding estimates of the genetic parameters were obtained. Within each production systems, genetic progress was estimated for each population assuming selection of the 15% superior HSF and subsequent recombination. Results showed that the experiments did not reveal sufficient genetic variation, in both populations, able to be explored in selection programs under water deficit conditions and also a low concordance level between the performance of the families under normal irrigation and water deficit.