OBJECTIVE:To investigate the protective effects of Zuogui Wan (左归丸,ZGW) on bone loss induced by ovariectomy (OVX) and its mechanism via orex-in-A and orexin receptors in the osteoporosis rat model.METHODS:Fifty Sprague-Dawley female rats were randomly divided into sham-operated (sham)group and four OVX subgroups.Rats subjected to sham and OVX were treated with the vehicle(OVX,1 mL/100 g weight,n =10),17β-estradiol(E2,50 μg· kg-1· d-1),and ZGW at the doses of 2.3(ZGW-L) and 4.6 (ZGW-H) g/kg/day lyophilized pow-der daily for 3 months,respectively.The serum bio-chemical parameters of 17β-estrogen (17β-E2),tar-trate-resistant acid phosphatase (TRACP-Sb) and bone alkaline phosphatase (BALP) were measured by enzyme-linked immunosorbent assay.Hematox-ylin-eosin staining was used to detect the changes in the morphological structure in bones.Microcom-puted tomography wasused to evaluate the bone mineral density and microarchitecture of the distal femur.The gene or protein expression of orexin-A,orexin receptor 1 (OX1R),orexin receptor 2 (OX2R),osteoprotegerin (OPG) and receptor activator of nu-clear factor-κ B ligand (RANKL) were assayed by ei-ther quantitative polymerase chain reaction or Western blot analysis.RESULTS:Compared with the OVX group,ZGW could reduce the serum level of TRACP-Sb and in-creased the serum levels of BALP and17β-E2 (P ＜0.01).Meanwhile,ZGW could prevent bone loss and improved bone trabecular microarchitecture by increasing the trabeculae structure thickness and trabecular number,and arranging the trabecu-lae structure properly.Compared with the OVX group,it was upregulated for the orexin-A and OX2R mRNA or protein expression from the hypo-thalamus and tibiae,and OPG in the tibiae of ZGW groups (P ＜ 0.01,＜ 0.05),while downregulated for the OX1R mRNA and protein expression in the tibi-ae and hypothalamus and RANKL from the tibiae(P ＜ 0.01).CONCLUSION:ZGW exhibited a protective effect for PMOP that may be mediated via orexin-A and orexin receptors regulation.