The objectives of this study were to determine maize populations and population crosses of high tolerance to low-N and to study heterosis and combining ability of diallel population crosses under low- and high-N conditions. In 2002 season, nine maize open-pollinated populations were crossed in a diallel system (excluding reciprocals). In 2003 season, the 9 parental populations and resulting 36 population crosses were evaluated under high- and low-N conditions. Significant mean squares were existed among studied genotypes and N levels for all studied traits, except for ears/plant. Estimates of heterobeltiosis for grain yield and its components increased under low- than under high-N conditions. Ten out of 36 population crosses showed significant positive heterobeltiosis for grain yield under low-N. The highest positive heterobeltiosis estimates for grain yield and nitrogen use efficiency(NUE) resulted from crossing between parent populations of different origins. The magnitude of GCA variance was greater than that of SCA variance for 6 traits including grain yield and NUE under both high- and low-N, indicating that additive was more important than non-additive genetic variance. Tuxpeno, Giza-2, DTP-1 and Tep-5 were the best populations in per se performance and GCA effects for grain yield and NUE and could be considered as suitable materials for improving traits related to low-N tolerance in maize selection programs. Superiority of population crosses in their per se performance, heterobeltiosis and SCA effects, were shown by the crosses Tep-5 X BS-26, C-87 X Tuxpeno , AED X BS-11 Giza-2 X Tep-5, Giza-2 X BS-26, C-87 X Tep-5 and Giza-2 X DTP-1 in descending order. Such population crosses could be recommended for a heterosis breeding program to isolate inbred lines and develop single cross hybrids of high tolerance to low- N stress conditions.