Sleep is regulated in part by the brain cytokine network including tumor necrosis factor alpha (TNF). TNF activates the transcription factor nuclear factor kappa B, which in turn promotes transcription of many genes including cyclooxygenase-2 (COX-2). COX-2 is in the brain and is an enzyme responsible for the production of prostaglandin D₂. The hypothesis that central COX-2 plays a role in the regulation of spontaneous and TNF-induced sleep was investigated. Three doses (0.5, 5 and 50 µg) of NS-398, a highly selective COX-2 inhibitor, were injected intracerebroventricularly (icv). The highest dose decreased non-rapid eye movement sleep (NREMS). The intermediate and highest doses decreased electroencephalographic slow-wave activity (EEG SWA), the greatest reduction occurred after 50 µg of NS-398 during the first 3 hour postinjection period. REMS and brain temperature (Tbr) were not altered by any dose of NS-398. Pretreatment of rabbits with 5 or 50 µg of NS-398 blocked the TNF-induced increases in NREMS, EEG SWA and Tbr. These data suggest that COX-2 is involved in the regulation of spontaneous and TNF-induced sleep.