In the immune system, interleukin (IL)-1β effects are mediated by the type I IL-1 receptors (IL-1RI), whereas the type II IL-1 receptors (IL-1RII) act as inhibitory receptors. IL-1RI and IL-1RII are also present in the brain. To study their functionality in the brain, mice were centrally treated with neutralizing monoclonal antibody (MAb) directed against IL-1RI (35F5, 1 μg) or against IL-1RII (4E2, 2 μg) and were centrally injected with recombinant rat IL-1β at a dose (2 ng) that decreased social exploration. Only 35F5 was effective in abrogating the behavioral effect of IL-1β. Moreover, 4E2 (1 μg icv) did not potentiate the behavioral response to a subthreshold dose of IL-1β (1 ng icv). To examine the ability of brain IL-1RI to mediate the effects of endogenous IL-1β, mice were centrally treated with 35F5 (4 μg) and peripherally injected with IL-1β (1 μg). Like IL-1 receptor antagonist (4 μg icv), 35F5 abrogated the effects of IL-1β. These results suggest that brain IL-1RI mediates the behavioral effects of IL-1β in mice.