Film autoradiography was used to demonstrate the transport and sites of accumulation of blood-borne radioiodinated interleukin-1 alpha (II-1 alpha) and other cytokines into the brain after intravenous administration. [125 I]Il-1 alpha, [125I]Il-1 beta, [125I]interleukin-1 receptor antagonist (II-1ra), and [125I]tumor necrosis factor-alpha (TNF alpha) labeled the choroid plexus and the capillary network 30 min after injection into the blood, suggesting that these areas may serve as sites of blood-to-brain transport. [125I]Il-1alpha, but not [125I]Il-1beta, [125I]Il-1ra, [125I]TNF alpha, or [125I]interleukin-2 (Il-2), was also found localized to the caudal region of the septal nuclei. Only unlabeled II-1 alpha was able to inhibit this accumulation. These findings provide further evidence for the passage of select cytokines across the blood-brain barrier (BBB) and are the first to identify a target site within the central nervous system (CNS) for a transported cytokine.