Objective The study was carried out to investigate the effects of different dietary Ca levels on bone metabolism,based on the osteoprotegerin (OPG)-receptor activator of NF-κB ligand (RANKL)-receptor activator of NF-κB (RANK) system in growing WHBE rabbits.Methods Twenty one weaned male WHBE rabbits at the age of 42 days were divided into 3 groups (I,Ⅱ and Ⅲ) according to dietary Ca levels (0.95%,1.10%,and 1.30%,respectively) for a 42-d feeding trial.The above three diets had similar phosphor (P)(about 0.64 g/kg),digestible energy (9.50 MJ/kg),crude protein (about 19.70%) and crude fibre (13.57%) contents.When the feeding trial finished,the serum indices of bone metabolism (Ca,PTH and BALP) were detected,and the OPG-RANK-RANKL system in bone tissue was analyzed by real-time fluorescence quantitative PCR assay and immunohistochemistry,respectively.At last,the relationships between bone metabolism and dietary Ca were evaluated according to RANKL/OPG ratio.Results All the contents of serum Ca,PTH and BALP had no significant differences in the groups I,Ⅱ and Ⅲ (P>0.05).Both the RANKL mRNA and RANKL/OPG mRNA ratio were lowest in the group Ⅱ and had significant differences with group I and Ⅲ (P<0.05).Dietary Ca levels had significant effects on the protein expression of OPG-RANK-RANKL in bone tissues (P<0.01).The positive index of OPG in the groups Ⅱ and Ⅲ was significantly higher than that in the group I (P<0.01),while the positive index of RANK in the group Ⅱ was lower than those of the group I and Ⅲ (P<0.01).The protein expression positive index ratio of RANKL to OPG was also lowest in the group Ⅱ,showing a significant difference with group I (P<0.01).Furthermore,both the gene transcription ratio of RANKL to OPG and the protein expression positive index ratio of RANKL to OPG had significant correlations (with quadratic curve) to the dietary Ca levels (R²=0.4068,0.8433;P<0.05,P<0.001).Conclusions In summary,the bone metabolism of WHBE rabbits during growing periods has significant correlation with dietary Ca levels.An optimal bone metabolism status can be obtain at 1.10% dietary Ca level as demonstrated in this study.