The recent purification and characterization of an interleukin 1-receptor antagonist (IL-1ra) has provided an additional means of elucidating the mechanisms involved in the responses initiated by IL-1. Central administration of IL-1 to rabbits results in a characteristic febrile response and in increased non-rapid-eye-movement sleep (NREMS). In this study, rabbits received various doses of IL-1ra (10-1,000 micrograms) or pyrogen-free saline intracerebroventricularly, and sleep-wake activity and brain temperature (Tbr) were determined for the next 24 h. All doses of IL-1ra tested tended to reduce NREMS in the first postinjection hour with little effect on Tbr. When rabbits were pretreated with 100 micrograms IL-1ra and then injected with 10 ng IL-1, the characteristic IL-1-induced febrile and NREMS-promoting effects were completely blocked.