Misonidazole is a known hypoxic cell sensitizer that binds covalently in hypoxic cells. Its congeners labeled with 77Br, 75Br, or 18F, are likely candidates for imaging hypoxia. We have synthesized and tested [82Br]-4-bromomisonidazole, [3H]-4-bromomisonidazole, [3H]fluoromisonidazole and [3H]misonidazole as prototype radiopharmaceuticals and have compared their uptake in normal and malignant tissues. The higher lipophilicity of brominated misonidazole increased its concentration in the hypoxic portion of tumors at 2 hr, but high blood levels contributed to excessive background, incompatible with imaging. Hydrogen-3-fluoromisonidazole diffused into tumors at a slower rate than misonidazole but it also cleared from normal tissues so that after 2 hr tumor-to-blood ratios favorable for imaging were achieved. In the compounds that were studied, fluorine at the end of the alkyl chain is more stable in vivo than bromine on the imidazole ring. Our results indicate that [18F] fluoromisonidazole may be a useful tracer for imaging hypoxia at approximately 4 hr after injection.