REM sleep in the human declines from about 50% of total sleep time (~8 hours) in the newborn to about 15% of total sleep time (~1 hour) in the adult, and this decrease takes place mainly between birth and the end of puberty. We hypothesize that, if this developmental decrease in REM drive does not occur, lifelong increases in REM sleep drive may ensue. In the rat, the developmental decrease in REM sleep occurs between 10 and 30 days after birth, declining from over 70% of total sleep time in the newborn to the adult level of about 15% of sleep time during this period. Rats aged 12-21 days were anaesthetized with Ketamine, decapitated and brainstem slices cut for intracellular recordings. We found that excitatory responses of pedunculopontine nucleus (PPN) neurons to NMDA decrease, while responses to kainic acid increase, over this critical period. Serotonergic type 1 agonists have increasing inhibitory responses, while serotonergic type 2 agonists do not change, during this developmental period. The results suggest that, as PPN neurons develop, they are increasingly activated by kainic acid and increasingly inhibited by serotonergic type 1 receptors. These processes may be related to the developmental decrease in REM sleep. Developmental disturbances in each of these systems could induce differential increases in REM sleep drive, accounting for the post-pubertal onset of a number of different disorders manifesting increases in REM sleep drive. Examination of modulation by PPN projections to ascending and descending targets revealed the presence of common signals modulating both ascending arousal-related functions and descending postural/locomotor-related functions.