Nipple-shifting behavior was studied in rats 3--30 days old in 4 experimental paradigms. The incidence of nipple-shifting of rats tested in groups of 3 on their nonlactating, anesthetized mother was age-related. Rats 12 days of age and younger did not leave the nipple first suckled during the 2-hr test period. Starting by Day 15, however, nipple-shifting increased and reached its maximum in 24-day-old rats. This behavior's incidence was directly related to maternal (and, therefore, nutrient and water) deprivation (Experiment I). Milk letdown reduced the incidence of nipple-shifting behavior at all ages studied and synchronized its occurrence, as almost all shifts occurred immediately aftet letdown and almost none during the 15-min interval between successive milk letdowns (Experiment II). Testing rats individually on the nonlactating, anesthetized mother produced age-related effects. Shifting was virtually eliminated in 15-day-old rats, markedly reduced in 21-day-old rats, and not affected in 27-day-old rats tested individually (Experiment III). Rats 27 and 30 days of age, upon leaving a nipple, ate and did not return to suckle. Rats 15 days old never ate and always returned to suckle (Experiment IV). Twenty-one-day-old rats suckled, and many ate in the mother's presence. The significance of these findings relates to maximizing milk intake and facilitating the process of weaning.