Infants acquire independent mobility amidst a flux of body growth. Changes in body dimensions and variations in the ground change the physical constraints on keeping balance. The study examined whether toddlers can adapt to changes in their body dimensions and variations in the terrain by loading them with lead weights and observing how they navigated safe and risky slopes. Experiment 1 verified the reliability of a new psychophysical procedure for testing infants' responses in 2 experimental conditions. In Experiment 2, this procedure was used to compare infants' responses on slopes in feather-weight and lead-weight conditions. The lead weights impaired infants' ability to walk down slopes. Babies adapted to altered body dimensions by treating the same degree of slope as safe in the feather-weight condition but as risky in the lead-weight condition. Exploratory activity on the starting platform predicted adaptive responses on risky slopes. Locomotion requires continual adaptation of ongoing move- ments. Changes in body dimensions and variations in the proper- ties of the ground surface change the physical constraints on maintaining balance. The present research examined whether walking infants can adapt to experimental manipulation of their body dimensions to cope with locomotion over varied terrain.