The emergence of behavioral states is one of the most significant aspects of development. The rat is very immature at birth and in some structural and functional aspects of CNS (central nervous system) development it is comparable to a 7-month-old human fetus. At this stage of development synchronization of different state criteria is poorly organized. Infant rats spend very little time in wakefulness and, once asleep, they still display a very high level of motor activation, with frequent rapid eye movements and uncoordinated myoclonic jerks. Although it is questioned whether the activated state of sleep in the newborn rat is comparable to rapid-eye movement sleep (REM) in adults, it has been shown that the CNS displays an increased level of endogenous neuronal activation even in very immature animals during this state. To study the functional significance of REM in early life, rat pups were deprived of this state from 1 to 3 wk of age and tested as adults. In the rat, chronic suppression of REM by interfering with monoamines during early development induced hyperactivity, hyperanxiety, attentional distractability, sleep disturbances, reduced sexual performance and reduced cerebral cortical size. In studies using instrumental, surgical or other pharmacological treatments to suppress REM similar effects on the development of brain and behavior were found. Taken together, these findings point to a role for REM during early development, so that more attention should be given to the potential hazards of medicines (and/or pathologic conditions) which induce reduced levels of REM and or disturbed monoamine activities in the brain during late prenatal and early postnatal life.