OBJECTIVE:To investigate the protective effects of Zuogui Wan(左归丸, ZGW) on bone loss induced by ovariectomy(OVX) and its mechanism via orexin-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 powder daily for 3 months, respectively. The serum biochemical parameters of 17β-estrogen(17β-E2), tartrate-resistant acid phosphatase(TRACP-5 b) and bone alkaline phosphatase(BALP) were measured by enzyme-linked immunosorbent assay. Hematoxylin-eosin staining was used to detect the changes in the morphological structure in bones. Microcomputed tomography was used 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 nuclear factor-κB ligand(RANKL) were assayed by either quantitative polymerase chain reaction or Western blot analysis.RESULTS:Compared with the OVX group, ZGW could reduce the serum level of TRACP-5b and increased the serum levels of BALP and 17β-E2(P < 0.01, < 0.05), while downregulated for the OX1R mRNA and protein expression in the tibiae 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.