We report here the ontogeny of a circadian clock of the field mouseMus booduga expressing itself 16 days after parturition in the locomotory activity of neonate pups removed from the mother and held in continuous darkness ever since birth. Locomotion is a ‘complex’ activity serving such functions as foraging, exploration, and territoriality. Since these functions are not conventionally associated with neonate and altricial animals, it is of interest that this ability has such an early circadian origin. A backward extrapolation of the pups rhythm and the rhythm of the mother strongly implicate maternal synchronization. The period of the circadian rhythm of the pups shortens with age, from birth up to six months.