No study has directly estimated tumor blood flow after antiangiogenic therapy. Using dye extraction colored microspheres, we estimated blood flow in a rat LY80 tumor model before and after antiangiogenic therapy with TNP-470. Dyed microspheres were infused into the tumor-bearing rats. The dye was extracted and its concentration was quantified by spectrophotometry. Tumor blood flow corrected for wet weight (ml. min(-1)g(-1)) was calculated as follows (AU = absorbency units): (blood flow to tumor) = (AU per gram of tumor) x (reference withdrawal rate)/(AU per gram of reference blood). Tumor tissues with or without TNP-470 were also examined histologically. Tumor blood flow 1 week after transplantation could be predicted by the simple regression equation. In the groups treated with TNP-470, this simple regression equation shifted to the left, despite virtually no difference in the results of pathological examination. In LY80 tumors 2 weeks after transplantation, blood flow in tumors treated with TNP-470 was higher than without TNP-470. This technique can be used to obtain information about the angiogenic status within tumor before and after antiangiogenic therapy. These angiogenic profiles may provide important clues to optimal antiangiogenic therapy, combinations of antiangiogenic treatment and cytotoxic therapy, and schedules for combination therapy.