A U-shaped function of behavioral growth characterizes the early ontogeny of face grooming in mice. In the first few postnatal days (0-100 hr), mice groom their face by using temporally isolated strokes, or bouts of strokes, which vary in amplitude and symmetry. Later on (100-200 hr), bouts disappear, asymmetry is eliminated, and the amplitude of strokes is gradually restricted; the infants engage in stereotyped, double-handed, temporally isolated strokes. Finally (200-300 hr), bouts reappear, including both short and long, symmetrical and asymmetrical, strokes. These changes, are accompanied by unidirectional changes such as an increasing participation of the trunk, the neck, and the head in grooming, which lead to the flexible organization of face grooming seen in adults. Instead of describing development in terms of emerging unitary "acts," we have recorded from high-speed films three simultaneous aspects of movement: the movements of the separate limb and body segments, the resultant paths which are traced by the forepaws, and the paths of contact which are traced on the face. This method of description discloses (a) reversible changes, and (b) a change in the relative stability of each of these aspects of face grooming, in the course of early ontogeny.