Summary The tritium-labelled analogues of pimonidazole and RSU 1069 were injected into mice bearing the KHT murine sarcoma which has a hypoxic cell fraction of 1-0%. The distribution of activity at 24 h was recorded using autoradiography and measurement of tissue activity. Autoradiographs with both drugs showed high activity in particular cells within tumour, eye (melanin-associated cells), eyelid (Meibomian gland), liver (centrilobular area), skin (sebaceous gland and melanin), stomach (squamous area), footpad, oesophagus, labial gland, Zymbal's gland, preputial gland, parotid gland (intralobular ducts) and airway epithelium. These tissues had previously been identified as sites of binding of misonidazole. The measurement of total tissue radioactivity showed significantly higher activity in liver, eyelid (Meibomian gland), oesophageal lining, kidney and labial gland than was found in the tumour.Misonidazole (1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol; MISO) sensitises hypoxic tumour cells to ionising radiation (Asquith et al., 1974). This radiosensitisation is associated with the electron affinity of, MISO and is a fast (subsecond) free-radical process. MISO can also be involved in a separate, slower, enzyme dependent process in which, in the presence of the appropriate reductase(s), the nitro group is reduced yielding cytotoxic species (Varghese et al., 1976;Rauth, 1984). This second process is favoured by a hypoxic environment -such as occurs in parts of many tumoursand this leads to locally enhanced retention. The concept of the preferential retention of reactive bioreduced metabolites in tumours has generated two lines of research. One is directed towards tumour therapy (bioreductive drug therapy). In this, drugs are being developed which, as with MISO, on reduction in the hypoxic milieu of the tumour, form cytotoxic metabolites(s) that bind to critical macromolecules such as DNA. The other line of research is in the field of imaging and spectroscopy.