Tirapazamine, a new bioreductive agent currently advancing through clinical trials, may have a valuable role to play in cancer therapy. In vitro, the drug shows markedly more toxicity to hypoxic cells than to aerobic cells, and preferential activity against hypoxic cells of solid tumors in vivo also can be inferred in many investigations. However, we have previously reported that tirapazamine has minimal activity against cells in the center of hypoxic spheroids, raising concerns with regard to whether the drug may be bioreductively inactivated before reaching chronically hypoxic tumor cells. We consequently examined the oxygen-dependent differential activity of tirapazamine in solid tumors in vivo by using fluorescence-activated cell sorting with clonogenicity assays for cell viability or with the comet assay for DNA damage. The preferential activity of tirapazamine against hypoxic vs. aerobic tumor cells in vivo was approximately threefold, much less than the factors of 50-500 typically seen in vitro. Interestingly, we also found that tirapazamine administration often modified tumor blood flow in the murine models, an effect that could be of clinical utility in sufficiently sensitive tumor cells. Taken together, our observations suggest that sequencing of tirapazamine with other agents requires careful consideration in the clinic.