¡¾Abstract¡¿ AIM: To investigate the detailed innervation patterns of the sympathetic nerve to the cochlear blood vessels and related neurotransmitters in the guinea pig. METHODS: The spiral modiolar artery(SMA) in guinea pig cochlea was exposed by microsurgery, then the fluorogold(FG) or horseradish peroxidase(HRP) was given around the SMA for retrograde tracing. The bilateral superior cervical ganglia(SCG) and stellate ganglia(STG) were observed to find retrogradely labeled neurons. The neurotransmitters of these sympathetic fibers around SMA were ascertained by using retrograde tracing combined with immunohistochemical doubleª²labeled technique. RESULTS: Retrogradely labeled neurons with FG or HRP were found in the rostral half of the ipsilateral SCG. No labeled neurons were observed in the contralateral SCG and the bilateral STG. Most of HRPª²labeled neurons in SCG were THª²IR positive and NPYª²IR positive, while none of them were CGRPª²or SPª²positiue. CONCLUSION: The THª²IR positive and NPYª²IR positive neurons in SCG send projections to the ipsilateral cochlear blood vessels. The norepinephrine and NPY may be used by sympathetic nerve to regulate cochlear blood flow. ......