Abstract Interleukin-6 (IL6) induces acute phase protein production and is hypothesized to mediate systemic and central effects of IL1. To determine whether IL6 possesses somnogenic properties, rabbits were injected intracerebroventricularly with IL6; sleep-wake activity was determined and brain temperatures recorded for 6 hr. IL6 induced fever in a dose-related manner with no effect on sleep-wake activity. IL6, therefore, is the first cytokine reported to elicit fever without promoting sleep. We conclude that the somnogenic action of IL1 is not mediated through IL6.