Interleukin-1beta (IL-1beta) is a cytokine that contributes to the hyperalgesia, inactivity, and fever associated with illness. These three components of the illness response occur simultaneously following peripheral administration of IL-1beta. The objective of the present study was to determine whether hyperalgesia, inactivity, and fever correspond following central administration. Rats were injected with IL-1beta (0.05 pg-50 ng/10 microl) into the lateral ventricle and core body temperature and activity were assessed for 5.5 h using radio telemetry while rats remained in their home cage. Rats were removed from the cage periodically to assess nociception by measuring the latency for hindpaw withdrawal to radiant heat. The two highest doses of IL-1beta (5 and 50 ng) caused an increase in core body temperature and a decrease in activity beginning 105 min following administration. No change in nociception was evident at any time after administration of IL-1beta regardless of dose. These data indicate that the hyperalgesia associated with fever is triggered by a peripheral, not a central action of IL-1beta, presumably by activation of vagal afferents.