We investigated the influence of the sleep/waking cycle, the effects of paradoxical sleep deprivation (PSD) and of the vigilance-promoting drug modafinil on the amino acid contents of rat brain cortex. No significant nycthemeral variations in amino acid levels could be detected. PSD (12-24 hours), using the water tank method, significantly increased the levels of glutamate and glutamine. The increase was still observed after the sleep rebound period. gamma-Aminobutyric acid (GABA) levels did not change significantly during the instrumental sleep deprivation but increased during the rebound period. Control experiments indicate that the increase in glutamate and glutamine levels is due to PSD rather than to the stress associated with the experimental procedure. The increase in glutamate content cannot arise only from transamination reactions, because the levels of other amino acids (such as aspartate) did not decrease. Modafinil treatment did not significantly modify the brain cortex content of any of the amino acids tested.