Cortical arousal was altered by electrical brain stimulation (EBS) to the mesencephalic ascending reticular formation beginning at 27 days of age and administered 2 h/day for either 14 or 28 days. Subjects receiving brain stimulation from 27 to 41 days of age showed a significant decrease in telencephalon AChE activity and weight when compared to controls. These decreases in biochemical and physical parameters corresponded to a marked decrement in maze performance. The advantages and potential uses of the weanling chronic EBS preparation to investigate critical developmental periods are considered.