Apnea of prematurity in the majority of the cases becomes evident during the nonREM phases of sleep. Aminophylline, one of the most commonly used xantines in prevention, seems to reduce the incidence of these crises, increasing the duration of the REM phase and of the wakefulness of the newborn. It is possible that the serotonin, neuromediator of the REM phase, represents an important element in determining the crises of apnea, conditioned by variation in the serum levels of tryptophan that regulates the synthesis and liberation of cerebral serotonin. Our data demonstrate how aminophylline increases the post-natal physiological diminution of total tryptophan in the preterm. Therefore we can deduce, from our results, that an increased synthesis and liberation of serotonin may to increase the duration of the REM and to reduce the incidence of apnea.