Chronic exposure to lead during development is associated with cognitive dysfunction in children and animals and impairment of release of neurotransmitters in the brain. Some amino acid neurotransmitters in the CNS are critical for the induction of LTP, which is considered a potential mechanism of learning and memory. In this study, the extracellular levels of amino acids in the dentate gyrus (DG) of the hippocampus of early postnatal rats exposed to lead were measured by in vivo microdialysis, before and after 50 days of training. Samples of cerebrospinal fluid were analyzed by high-pressure liquid chromatography (HPLC) and fluorescence detection. Compared to pre-training, the concentration of glutamate in the post-training samples increased by 164.2 and 222.6% in the control and lead-exposure rats, respectively. After training, the extracellular concentration of GABA and glycine decreased by 49.4 and 44.3% in lead-exposed rats, respectively, whereas in the after-training samples of control rats, the concentration of GABA was unchanged and glycine decreased by 21.8%. The results of this study may suggest that concentrations of the neurotransmitters were changed during the learning process and lead impaired the neurotransmitter systems, especially glutamate and GABA systems.