We previously reported that a novel combretastatin A-4 derivative, AC7700, has remarkable antitumour effects because of an irreversible stasis of tumour blood flow (TBF) and subsequent loss of nutrient supply to tumour tissue. Since early 2002, under the new designation AVE8062, AC7700 has undergone clinical trials in Europe and the US. Questions remain, however, concerning how AC7700 blocks TBF and why the TBF stasis does not recover. In this study, using a rat tumour LY80, a variant of Yoshida sarcoma, we examined whether TBF cessation after AC7700 administration is due to a direct action of the agent on tumour blood vessels. We constructed electrodes that can drop a small quantity of the drug solution directly at the site of blood flow measurement and inserted them subcutaneously and into the tumour. We compared the blood flow responses of normal vessels and tumour vessels after administration of 10-μl doses of various concentrations (0.2, 1, 10, and 50 mg ml− 1) of the AC7700 solution. In addition, we assessed TBF stasis after iv and intra-arterial 10 mg kg− 1 AC7700 administration in an LY80-induced kidney tumour. To determine why the TBF stasis is irreversible, we observed AC7700-induced changes in host arterioles and the tumour vascular network of the Sato lung carcinoma using a vital microscopic rat transparent chamber. Since an increase in tumour interstitial fluid pressure brings about a decrease in TBF, we also measured 10 mg kg− 1 AC7700-induced changes in this pressure. The sensitivity of the blood flow response after intratumoral application of AC7700 was markedly higher in normal vessels relative to tumour vessels. Intra-arterial administration of AC7700 did not have stronger effects on TBF stasis than did iv administration. Intravital microscopy showed that AC7700 induced a powerful and long-lasting constriction of host arterioles, so that complete stasis of blood flow occurred in downstream vessels, which supplied blood to tumours. Owing to this stasis, the lumens of numerous tumour vessels narrowed or completely disappeared, and numerous erythrocytes stagnated in drainage vessels of the tumour vascular network. Haemolysis of these erythrocytes occurred after 2–3 h, resulting in complete thrombosis. There was no indication of reperfusion in vessels showing haemolysis. This haemolysis is thought to be the main cause for the irreversibility of TBF stasis. Since the tumour interstitial fluid pressure decreased after iv AC7700 administration, the possibility of stasis of TBF being caused by tumour vascular compression was excluded. All these results strongly suggest that the main target of AC7700 is host arterioles and that the stasis of TBF induced by AC7700 is not triggered by a direct action of the drug on tumour vessels.