Objective To investigate the effect of the histological changes on rat dorsal root ganglia (DRG) after 125I seed brachytherapy.Methods Twelve adult male Sprague-Dawley rats ( 150-180g each) were randomly divided into 6 groups,125I seeds with different activities of 0 (Titanium shell),14.8,18.5,22.2,25.9 and 29.6 MBq were implanted to 6 groups of rats respectively and the behavioral changes of rats were observed.The rats were killed in different periods after implantation,the morphological changes in DRG and surrounding muscle tissue were observed with an Olympus BX51 optical microscope and then the irradiation doses were estimated.Results After 125I seed implantation,the movement function of rats was not affected and the weight of rats gained after 7 days.After the titanium shell implantation,very few mild swelling was induced in neuroganglion cells that still had clear nucleolus and normal cytoplasm.At 14 days after 18.5 MBq seed implantation,cell swelling was more serious and cell dehydrating,nuclear condensation and nuclear fragmentation appeared after 30 days.At 60 days after 29.6 MBq of seed implantation,nuclear dissolution and cytoplasmic shrinkage were induced in a large number of cells.In general,the severity of fibrosis was aggravated with the time post-irradiation and the dose in the muscles around the ganglion.Conclusions After 125I seed implantation,the injury degree of DRG tissue is dose-dependent,and the 125I seed irradiation would have analgesic effect on releasing intractable pain.