It is generally thought that fever is induced by blood-borne cytokines via an action on hypothalamic thermosensitive neurons. Recent studies suggest that the organum vasculosum laminae terminalis (OVLT), which lacks the blood-brain barrier, may be necessary for fever induction by systemic pyrogens. We have examined the responses of neurons in the OVLT to the pyrogenic cytokines, human recombinant tumor necrosis factor α (TNF) and interferon α2 (IFN), by recording extracellular single-units in brain slice preparations from guinea pigs. Of all the OVLT neurons tested with TNF (900–5000 ng/ml perfusate/min) and IFN (1200–8500 units/ml perfusate/min), administered for 2.5 min, 44% increased their firing rates (FR) for 30.6–57.5 min (average, 47.4 min) with onset latencies of 6.1–9.8 min (average, 7.9 min). The remaining 56% of the neurons did not change their FR after TNF or IFN. Carrier vehicles for these cytokines produced no FR change. The results suggest the possibility that the OVLT may be a site where chemical signals of blood-borne cytokines are transduced into neuronal signals.