The effects of intracerebroventricular injection (i.c.v.) of an interleukin-1 (IL-1) inhibitor, a soluble IL-1 receptor fragment (IL-1RF), on sleep and brain temperature (T(br)) responses of rabbits induced by mild increases in ambient temperature (T(amb)) were determined. Each rabbit was recorded under three conditions: (1) 21°C T(amb) plus pyrogen-free saline (PFS); (2) 27°C T(amb) plus PFS; (3) 27°C T(amb) plus the IL-1RF. The higher T(amb) significantly increased T(br) during the warming period; this effect was attenuated by pretreatment with the IL-1RF. The higher T(amb) alone (6 h exposure) significantly increased non-rapid eye movement sleep (NREMS) across the 23-h recording period. However, during the 6-h warming period NREMS values, obtained after IL-1RF treatment, were not significantly different from those obtained from PFS-treated animals at 27°C T(amb). The ability of the IL-1RF to block T(amb)-induced changes in T(br) and the failure of the IL-1RF to block T(amb)-induced NREMS responses is different from previous results which indicated that a tumor necrosis factor receptor fragment (TNF-RF) inhibits warm T(amb)-induced sleep but not T(br) responses. Thus, brain IL-1 and TNF sleep and thermo mechanisms are, in part, different.