Objective To study the effects of chronic manganism on hearing and cochlear cells in rats by using animal model of chronic manganism .Methods Sixty adult SD rats were randomly divided into Mn - exposed and controlgroups.RatsweretreatedwithMnCl24H2O(100mg·kg -1·d-1)ordeionizedwaterbygastricperfusion, lasted for 12 weeks .The Mn concentration in peripheral blood was measured respectively at 4 weeks ,8 weeks and 12 weeks after treatment .At 12 weeks after treatment ,the auditory brainstem response was recorded ,the hair cells morphology and counting were examined by stretched preparation of basilar membrane stained with FITC -phalloi-din ,and the spiral ganglion cells morphology and counting were studied by HE staining ,the ultrastructure changes of hair cells and spiral ganglion cells were detected by transmission electron microscopy .Results The blood Mn concentration increased gradually with time after treatment .ABR thresholds at 4 ,8 ,16 ,24 and 32 kHz were sig-nificantly increased at 12 weeks after treatment ,especially in the high-frequency range .Morphological study at 12 weeks after treatment showed loss of outer hair cells ,mainly in the basal turn of the cochlea ,and decreased number of spiral ganglion cells .The ultrastructure changes of outer hair cells and spiral ganglion cells included the break -ups ,disappearance or vacuolar change of mitochondria cristas .Conclusion Our data demonstrate that chronic man-ganism can cause loss of outer hair cells and spiral ganglion cells in cochlear in rats ,leading to hearing loss .