Aim Toinvestigatetheanalgesiceffectsof epidural osthole application on the mechanical allodyn-ia and the ERK/MAPK signaling pathway and the expression of COX-2 mRNA in the spinal dorsal horn.Methods 125adultmaleSDratswererandomizedin-to five groups( n=25 each) :Blank, Sham, NP, Ost and vehicle. At postoperative day 6, 1mg/rat osthole 50 μl was injected epidurally into group Ost and the same volume of vehicle was given into group vehicle. The mechanical pain threshold was measured by 50%MWT at 1 day before operation and the 3 rd,6 th,7 th, 14 th,21 st day after operation. After the measurement of pain threshold on postoperative day 14 , the L4-6 segment of spinal dorsal horn was removed for determi-nation of the expression of ERK, pERK and COX-2 mRNAbyWesternblotandRT-PCR.Results Com-pared with blank group, the mechanical pain threshold was only down-regulated at day 1 after operation in sham group, the expression of pERK and COX-2 mR-NA in sham group showed no significant difference ( P>0. 05 ); the mechanical pain threshold was signifi-cantly down-regulated after operation in NP, Ost and vehicle groups( P<0. 05 ) and the expression of pERK and COX-2 mRNA was significantly increased ( P <0. 05). Compared with vehicle group, the pain thresh-old in Ost group was significantly increased after drug administration( P<0. 05 ) and the expression of pERK and COX-2 mRNA was significantly reduced ( P <0. 05 ) . The expression of ERK showed no significant difference among each group(P>0. 05). The correla-tion analysis on pERK1/2 and COX-2 mRNA revealed the Pearson correlation coefficient was 0 . 878 and 0 . 910 , suggesting a strong positive correlation between pERKandCOX-2mRNA.Conclusions Ostholead-ministrated in the early stage after surgery can alleviate the nucleus pulposus-induced radicular inflammatory pain probably by inhibiting the expression of pERK and COX-2 mRNA in spinal dorsal horn.