Tumor necrosis factor (TNF) is a cytokine that possesses many biological activities, including enhancement of non-rapid-eye-movement sleep (NREMS). The role of endogenous TNF in the regulation of spontaneous sleep is unknown. If TNF is involved in sleep regulation, then reduction of endogenous TNF should suppress spontaneous sleep. A soluble TNF-binding protein I (TNF-BP I) and a synthetic fragment of TNF-BP I, TNF-R-(159-178), that contains the biologically active region of TNF-BP I, were used. These substances bind TNF and possess TNF-inhibitory activity; their effects on rabbit sleep after intracerebroventricular injection were determined across a 6-h recording period. Two doses of TNF-BP I (0.05 micrograms and 0.5 micrograms) were administered; the higher dose of TNF-BP I significantly decreased NREMS. Four doses of TNF-R-(159-178) (0.25 micrograms, 2.5 micrograms, 25 micrograms and 50 micrograms) were used. The 25 micrograms and 50 micrograms doses significantly suppressed NREMS. The highest dose (50 micrograms) also decreased REM sleep. These results are consistent with the hypothesis that endogenous brain TNF is involved in the regulation of normal sleep.