To compare the ability of combretastatin A-4 disodium phosphate (CA4DP) and 5,6-dimethylxanthenone-4-acetic acid (DMXAA) to change tissue blood perfusion. The tissues were a C3H mouse mammary carcinoma and various murine normal tissues, with perfusion measured using the 86 RbCl extraction technique. CA4DP (250 mg/kg; i.p.) reduced tumour perfusion to 34% of that seen in controls within 1 h of injection. It was maintained at this for at least 6 h, returning to control levels by 24h. This decrease was dose-dependent. DMXAA (25 mg/kg; i.p.) caused a 79% reduction in tumour perfusion 6 h after injection; no recovery was observed even after 24 h. DMXAA showed no changes at doses below 10 mg/kg. Both CA4DP and DMXAA increased perfusion in the gut, kidney, bladder and lung, while decreasing splenic perfusion. CA4DP tended to decrease perfusion in muscle, while DMXAA increased liver perfusion. These changes in normal tissue perfusion were generally less than those changes seen in tumours. No significant changes were seen in skin. CA4DP and DMXAA produced a selective and significant reduction in tumour perfusion, but the pattern of change was different. These results suggest how these vascular targeting drugs should be combined with more conventional therapies.