Tirapazamine (TPZ) is the lead member of a class of bioreductive drugs currently in phase II and III clinical trials. TPZ requires metabolic activation to give a cytotoxic free radical species, and this hypoxia-mediated process is carried out by a variety of cellular reductases, including NADPH cytochrome c (P450) reductase (P540R). Nitric-oxide synthase (NOS) is widely expressed in human tumors, and this enzyme consists of an oxidase and a reductase domain, the latter showing striking homology to P450R. Thus, in this article, we have investigated the role of one of the cytosolic isoforms of NOS [inducible NOS (NOSII)] in the bioactivation of this DNA-damaging antitumor agent. To achieve this, we have constitutively overexpressed NOSII in human breast tumor MDA231 cells by employing an optimized expression vector in which the strong human polypeptide chain elongation factor 1alpha promoter drives a bicistronic message encoding the genes for human NOSII and the puromycin-resistant gene (pac). Subcellular localization of NOSII in the stably transfected clones was determined after differential centrifugation and showed that NOSII catalytic activity was exclusively cytosolic as determined by conventional activity assay. This was confirmed by immunostaining followed by fluorescent microscopy studies. The increase in NOSII activity in a series of transfected clones was associated with an increase in TPZ metabolism and toxicity under hypoxic conditions. There was no similar increase in aerobic toxicity. These findings are of significance for two reasons. First, cellular NOSII activity, similar to that seen in human breast cancer, could contribute to TPZ toxicity; second, this will be a result of NOS-derived/cytosol-associated TPZ radicals.