Much evidence implicates interleukin-1beta (IL-1beta) in sleep regulation. Two previous studies indicated that levels of IL-1beta in mRNA were affected by sleep. In the current study, levels of IL-1beta mRNA and IL-1 receptor assessory protein (IL-1RAP) mRNA were determined 1 h after the beginning of light and dark periods and after sleep deprivation, using the reverse transcriptase-polymerase chain reaction (RT-PCR) and mutated internal standards. Daytime samples contained relatively more IL-1beta mRNA than nighttime samples, and levels of IL-1beta mRNA were higher after sleep deprivation. These changes occurred in the hypothalamus, hippocampus, cerebral cortex, and mesencephalon/pons. In contrast, the IL-1 RAP mRNA level did not seem to be affected by sleep.