Interleukin-6 (IL-6) can alter brain function after peripheral administration, suggesting that it, like IL-1α, IL-1β and TNF-α, might be able to cross the blood-brain barrier (BBB). We used multiple-time regression analysis to measure the unidirectional influx constant (K_i) into brain of radioactively labeled murine and human IL-6 given i.v. K_i values ranged from 3.05 to 4.54 (10⁻⁴) ml/g/min and were inhibited by unlabeled IL-6 but not IL-1α or TNF-α, showing that the transport system for IL-6 is distinct from those for IL-1α and TNF-α. ∼0.2% of the dose injected i.v. entered each gram of brain. The capillary depletion method showed that most of the IL-6 taken up by brain entered the parenchyma. However, only ∼16% of the radioactivity recovered eluted as intact I-IL-6 in brain and ∼50% in CSF after chromatographic separation by HPLC/Sephadex. The efflux rate for IL-6 injected into the lateral ventricle of the brain suggests that it enters the blood with the reabsorption of CSF. These results suggest that blood-borne IL-6 can reach sites behind the BBB, but that susceptibility to enzymatic dagradation may limit contact time within the CNS.