Aims: To investigate the toxic effect of gold nanoparticles (GNPs) 10 the learning and memory of mice. Main methods: Naked GNPs of 17 nm and 37 nm were synthesized, purified, and injected intrapcritoneally into BALB/C mice al a dose of 8 mg/kg/week. Passive avoidance test was performed to detect (he possible leaning impairment. Biochemical analysis to examine levels of monoamine and acetylcholine in mouse brain was also performed. ICP-MS was performed to examine the presence of GNP in brain. Localization of GNP at hippocampus was surveyed by ex vivo Coherent anli-Stoke Raman scattering (CARS) microscopy. Transmission electron microscopy (TEM) was also performed to verify the cellular location of GNPs at hippocampus. Key findings: Both 17 nm and 37 nm GNPs induced severe sickness in mice. However, only 17 nm GNP impaired the learning and memory of mice. GNP treatment elevated levels of dopamine from 114.5 ng/mouse brain to 143.6 ng/mouse brain for 17 nm (p<0.01) and to 138.2 for 37 nm GNP (p<0.05). Serotinin was significantly reduced by 17 nm GNP treatment from 572 ng/mouse brain to 44.3 ng/mouse brain (p<0.05). ICP-MS indicated the presence of GNPs in every part of the brain. CARS microscopy showed that 17 nm GNP was located at the Cornu Amonis regions of hippocampus where neuronal cells clustered, while 37 nm GNP was excluded from the cell clustered region. TEM and ED AX verified the presence of 17 nm GNP in the cytoplasm and in the dendrite of pyramidal cell, while 37 nm GNP was found in the extracellular region of neuronal cells. TEM image also indicated that both endocytosis and free diffusion coexisted for the invasion of 17 nm GNP. Significance: The current study provided evidence that nanoparticles were capable of entering into brain and affecting normal brain function. The size-dependent invading ability of GNPs provides an extra dimension for drug delivery.