The effects of frequency or duration of cyclic stress on the mechanical properties of collagen fascicles were studied by means of in vitro tissue culture experiments. Collagen fascicles of approximately in diameter were obtained from rabbit patellar tendons. During culture, cyclic stress having the peak stress of approximately was applied to the fascicles at for ( group), at for ( group), or at for ( group). The frequency of and the duration of are considered to be similar to those of the in vivo stress applied to fascicles in the intact rabbit patellar tendon. After culture for 1 or , the mechanical properties of the fascicles were determined using a micro-tensile tester, and were compared to the properties of non-cultured, fresh fascicles (control group) and the fascicles cultured under no load condition (non-loaded group). The tangent modulus and tensile strength of fascicles in the group were similar to those in the control group; however, the fascicles of the and groups had significantly lower values than those of the control group. There was no significant difference in the tensile strength between the and non-loaded groups, although the strength in the group was significantly higher than that of the non-loaded group. It was concluded that the frequency and duration of cyclic stress significantly affect the mechanical properties of cultured collagen fascicles. If we apply cyclic stress having the frequency and duration which are experienced in vivo, the biomechanical properties are maintained at control, normal level. Lower frequencies or less cycles of applied force induce adverse effects.