Objective To investigate the effect of cyclic tensile stress (CTS) on the metabolism inand apoptosis of rat chondrocytes. Methods Primary rat chondrocytes were cultured on a Bioflex plate for one day and then stretched cyclically for 24 hours at a frequency of 0.5 Hz using a Flexcell-5000T apparatus. The cells were divided into 5 groups according to their stretching ratio: 0% (the control group), 2%, 6%, 10% and 14%. After the stretching, Col II, Aggrecan, MMP-13 and ADAMTS-5 mRNA were measured using qPCRs, and the NO and PGE2 levels were measured using ELISA kits. Moreover, TUNEL staining and Annexin V-FITC/PI were used to analyze the apoptosis of chondrocytes. Results Compared with the control group, the average levels of Col II and Aggrecan mRNA decreasedin 10% and 14% groups [(0.738±0.11) and (0.58±0.13), (0.75±0.11) and (0.55±0.09)]. In those groups, the MMP-13 [(2.47±0.47) and (2.88±0.36)] and ADAMTS-5 mRNA level [(2.39±0.33) and (2.75±0.49)], the NO [(6.96±0.96) and (8.28±0.82)] and PGE2 level [(6.83±0.66) and (7.15±0.71)] had increased significantly. In the 6% group the average levels of Col II(1.76±0.30) and Aggrecan mRNA (1.93±0.14) of 6% group were significantly higher than the control group, but the NO level of the former (3.07±0.20) was significantly lower than the control group (3.89±0.33). The apoptosis rate of chondrocytesin 2% and 6% groups were (0.065±0.013) and (0.063±0.147), without significant differences to that of the control group (0.045±0.008). However, compared with the control group, apoptosis in the 10% and 14% groups [(0.135±0.026) and (0.184±0.020)] increased significantly. Conclusion The effect of cyclic tensile stress on chondrocyte metabolism and apoptosis was magnitude-dependent. Ten percent and 14% CTS can increase the catabolism and apoptosis of chondrocytes.Ten percent and 14% strain can increase the catabolism and apoptosis of chondrocytes.Cyclic 6% strain can increase the anabolism of chondrocytes, but 2% strain has no impact on metabolism or apoptosis. Key words: Cyclic tensile stress; Chondrocyte; Apoptosis; Metabolism; Rat