Abstract: Objective To explore the effects of brominated diphenyl ethers-209(BDE-209) exposure on spatial learning and memory abilities and ultramicrostructure of hippocampal neurons in adult rats.Methods Forty-eight Sprague Dawley adult rats were randomly divided into a solvent control group and three BDE-209-treated groups.The exposure groups were treated by intragastric administration of 250,500,and 1000 mg/kg BDE-209 once a day,respectively.The administration was lasted for 30 days.After the experiment,the capability of the learning and memory of the rats were measured with Morris water maze test.The specimens of hippocampus were sliced and morphological changes were observed under transmission electron microscopy.Results BDE-209-treated groups showed increased latencies compared with that of control group(P < 0.05).In the three BDE-209-treated groups,the latencies were increased with the increase of the dosage(P <0.05).The number of traversing the platform and the time staying in the quadrant of the original platform in four groups were 12.63±2.48,9.90±1.75,7.26±1.48,4.72±1.76 times and 52.92±7.78,31.89±5.97,24.82±6.65,18.25±5.23 seconds,with significant differences(P < 0.05 for all).The results also showed that the damages of the ultramicrostructure in hippocampal neurons were significant in the BDE-209-treated groups compared with that of the controls.Conclusion BDE-209 exposure induces structure and function damage in hippocampal neurons and the degression of learning and memory abilities of adult rats.