It has been shown that fibroblast growth factors (FGFs) protect hypocampal CAI cells from the effects of transient ischemia when injected either intraventrically or intravenously. The effectiveness of the systemic treatment seems to suggest that FGFs cross the blood-brain barrier to some extent. The appearance of basic fibroblast growth factor (bFGF) in the rat central nervous system after systemic administration have been autoradiographically examined using a 14C uniformly labeled preparation. Our results show that, two hours after a systemic bolus administration, bFGF spots in several populations of neuronal and in non-neural cells except in glial cells. Label accumulation was imperceptible when either 14C-bFGF was heated previously to the systemic injection or was co-administered with an excess of unlabeled bFGF. Our results indicate the existence of a saturable transport system of FGFs across the blood-brain barrier.