Three groups of rats (group a, b and c) with high and stable avoidance, and low variability, named Tokai High Avoiders (THA) strain. (a) via placenta from the 13th day of gestation, through maternal milk during lactation and per os after weaning (group Pb-gestation), (b) through maternal milk during lactation and per os after weaning (group Pb-birth), (c) per os after wearing (group Pb-weaning). The pre- and postnatal lead exposure induced apparent delays in speed of learning acquisition and broke the uniformity in avoidance rate in both males and females. Postnatal lead exposure caused slight delays in speed of learning acquisition in males alone. In the post wearing period defect in learning due to lead exposure could be observed in both males and females. No apparent difference was present in the development of pups among the lead exposed groups and control. From these results, it could be deduced that the most sensitive period of brain development in learning affected by lead exposure was the pre- and early postnatal period in the rat.