To determine if a moderate traumatic brain injury (TBI) sustained early in life alters the capacity for developmental plasticity, 17-20-day-old rat pups received a lateral fluid percussion and then reared in an enriched environment for 17 days. Compared to sham-injured controls, this moderate TBI prevented the increase in cortical thickness (1.48 vs. 1.68 mm, p < 0.01) as well as the corresponding enhancement in cognitive performance in the Morris Water Maze (39 vs. 25 trials to criterion, p < 0.05). These injured animals exhibited no significant neuronal degeneration and no evidence of neurologic or motor deficits. These findings strongly support the conclusion that a diffuse brain injury is capable of inhibiting both anatomical and cognitive manifestations of experience-dependent developmental plasticity.