The basal forebrain (BF) is an important mediator of cortical arousal, which is innervated by all ascending arousal systems. During sleep deprivation (SD) a site-specific accumulation of sleep factors in the BF results in increased sleep pressure (Kalinchuk et al., 2006; Porkka-Heiskanen et al., 1997; Porkka-Heiskanen et al., 2000). However, animals are able to stay awake and even increase their neuronal activity in the BF and cortex during SD, suggesting increased activity of the ascending arousal systems to counteract the effect of sleep pressure. This study used in vivo microdialysis to measure the effect of a 6 h SD, by "gentle handling" in freely moving rats, on the extracellular levels of serotonin and dopamine metabolites (5-HIAA, and DOPAC and HVA respectively) in the BF. Additionally, because glucocorticoids can interact with monoaminergic neurotransmission, and SD could be stressful, corticosterone levels were measured. We found an increase in extracellular serotonin and dopamine metabolite levels (n = 8, p ≤ 0.05). No interaction between corticosterone and the monoaminergic systems was apparent. Extracellular corticosterone levels showed no increase during the first 3 h of SD, and the subsequent increase (n = 8, p ≤ 0.05) did not result in values exceeding the normal diurnal maximum, indicating that no substantial stress was induced. The results demonstrate that SD increases extracellular dopamine and serotonin metabolites in the BF, suggesting increased activity of the ascending arousal systems. It remains to be investigated what the specific roles of the dopaminergic and serotonergic ascending arousal systems are in BF-mediated cortical arousal. © 2011 Elsevier B.V. All rights reserved.