The locomotor activity of the field mouse Mus booduga was monitored under continuous darkness (DD) for extended periods of 275 to 388 days. Free-running period (tau), activity time (alpha) and rest time (rho) were computed from the activity records for all the animals (n = 10). Decreases in the circadian period (tau) with age were observed in some of the animals at an early age (< ca. 100 days) and then tau remained relatively constant for rest of their life span. The activity time (alpha), rest time (rho), and their ratio (alpha/rho), however, did not show any consistent trend of increase/decrease. The initial decrease in tau is consistent with previous models suggesting a time-dependent increase in the number of cells generating oscillations in the suprachiasmatic nucleus (SCN) and/or increase in the strength of coupling between these cells. In such models, after an optimal number of cells launch themselves into a state of self-sustained oscillation, tau stabilizes, achieving relative constancy, a pattern similar to the one observed in our study.