A simple avoidance training procedure during early development produces massive neural traces in visual and somatic cortices of kittens reared in a normal environment. A preponderance of cells in these areas had response preferences for the stimuli used during training. Furthermore, some of these cells exhibited properties never found in normal animals not receiving such training. It appears that, even in an environment in which many other stimuli are present, some early experiences powerfully affect brain development and the way in which other experiences exert their effect.