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Introduction & Objective: Numerous observations in clinical and preclinical studies indicate that the developing brain is particularly sensitive to lead (PB)'s pernicious effects. The effects of low concentrations of lead on neurodevelopment are complicated. Lead acetate can disrupt both the CNS activity and neurons development. The present study was carried out to assess the effect of low level lead exposure on learning and memory by active avoidance learning. Materials & Methods: This experimental study was conducted at the Islamic Azad University of Parand in 2008. Eight groups of NMRI rats (9 rats in each group) (weight 220±30 gr) consisting of six experimental groups (3 after infancy and 3 adult groups) were exposed to low concentrations of lead for 45 days. The drinking water of the experimental groups was replaced by 0.05 %, 0.1 % and 0.2 % of lead acetate solution whereas the two control groups received distilled water. The results were analyzed using the SPSS software and student t-test. Results: In this study, the learning and memory tests showed no significant differences between experimental groups (infancy and adulthood) and infancy control and adult control in number of shocks for 0.05% concentration of lead acetate. The memory test showed an increase in number of shocks for 0.1% and 0.2% concentration of lead acetate in adult groups and an increase in number of shocks for 0.2% concentration of lead acetate in infancy groups (P<0.05). The learning test showed an increase in number of shocks for 0.2% concentration of lead acetate in infancy groups (P<0.05). Conclusion: Mechanisms of lead poisoning in the CNS are not clear and it as been suggested that lead exposure during life alters the granule cell neurogenesis and morphology in the hippocampus of infant or young adult rats.