Exotic maize (Zea mays L.) inbreds were introgressed into two elite Brazilian populations -BR105 (Suwan 1) and BR106 (related to Tuxpeno) to improve their yields and their breeding values at high plant density. Different traits were observed in a set of S1 families were observed per se at 50.000 pl/ha, and in top crossed to an inbred tester at 75.000 pl/ha). Introgression impact was estimated on these traits. Means, genetic variances, expected progresses and correlated responses were estimated for four key traits : yield, earliness, stalk crushing strengh, and root strength. Based on topcross performances, introgression brought in BR105 a slight yield increase with no influence on flowering dates whereas in BR106 a yield decrease and an earlier flowering were recorded. In both cases, ear rot drastically increased and root strength was reduced. Stalk strength tended to increase. Introgression reduced the genetic variances and the expected progress. The reduction of yield genetic variances might be due to changes in linkage equilibrium and epistatic interactions.