Tumour response depends on intratumoural cytotoxic concentration, which varies with tumour vascularity. We determined whether basic fibroblast growth factor (bFGF) infusion increased tumour vascularity, blood flow and cytotoxic drug uptake. The effect of interstitial and systemic bFGF infusion was compared with that of saline-infused controls using animal HSN and K12/TR tumour models. Changes in tumour vascularity were assessed by immunohistochemical staining of tumour sections. Blood flow and drug uptake were studied using a radiotracer method. There were significant increases in tumour vascularity, vessel length density and blood flow with both interstitial and systemic bFGF infusions, and a significant increase in tumour fluorouracil uptake after systemic bFGF infusion of liver tumours. The effects were independent of tumour type, and could be produced by bFGF administration after initial tumour growth. bFGF infusion increased tumour fluorouracil uptake. Further studies are required to determine the risks and benefits with this approach to increasing tumour cytotoxic uptake.