Misonidazole (MISO) is a nitroimidazole drug currently undergoing clinical trials as a radiosensitizer of hypoxic tumour cells. The drug is also toxic to such cells, probably because of metabolic activation of the nitro group under hypoxia. The metabolic fate of 14C-labelled MISO is examined, using hypoxic mammalian (CHO) cells in vitro. Organic-soluble and acid-soluble metabolites are formed, and radioactivity is bound to macromolecules. The organic-soluble products are separated by TLC and HPLC. Evidence is presented to show that one of the metabolites is hydroxylamino-misonidazole. The significance of metabolic nitroreduction is discussed.