A comparative analysis is presented which validates the practice of measuring free‐running period (r) of circadian locomotor activity by means of the slope of a line eye‐fitted to daily onset of activity. The slope of a linear regression calculated through daily onset of activity gives virtually the same ras the slope of the eye‐fitted line. The center‐of‐gravity (COG) of locomotor activity (defined as the mean daily clock time of all recorded activity impulses) is uniquely defined and free of subjective evaluation. COG is shown to be a meaningful representation of the central tendency of circadian activity; calculation of a regression through daily COG of activity also provides an accurate measurement of #GT.