The purine nucleoside adenosine may facilitate sleepiness by inhibiting neurons of the magnocellular basal forebrain that are known to enhance cortical activation during wakefulness. In vivo microdialysis sample collection in rats coupled to microbore high performance liquid chromatography analysis determined extracellular basal forebrain adenosine levels across the 24 hours of the day (lights on/off 07:00/19:00). During the dark period, when rats are predominantly active and awake, adenosine levels were elevated two-fold. Also, basal forebrain adenosine increased during 6 hrs of sleep deprivation and declined slowly thereafter. The data support the hypothesis that basal forebrain adenosine levels may reflect the homeostatic sleep drive that occurs daily in animals exhibiting a diurnal pattern of sleep and wakefulness.