The period of free-running rhythm was measured with two different devices, Automex and running wheel, in blinded female rats. The period was significantly shorter when measured with a running wheel than with an Automex. After transfer between the two devices, all 13 rats examined showed the same direction of change in the free-running period and that transfer from the Automex to running wheel shortened the period, while transfer from the running wheel to Automex elongated it, with the exception of two rats who did not show any significant change in the period even when they were transferred twice. These results indicate that free access to a running wheel shortens the free-running period in female blinded rats.