AbstractThe KHT transplantable tumour of C3H mice has been used as a model tumour for thein vivostudy of hypoxic cell sensitizers. Eleven sensitizers comprising four nitrofuran five nitrobenzene and two nitroimidazole derivatives, which have been shown to be effective on hypoxic mammalian cellsin vitro, have been investigated. Two of these compounds, metronidazole (2-methyl-5-nitroimidazole-1 ethanol) and tinidazole (ethy1 [2-(2′-methyl-5′-nitro-1′- imidazolyl) ethyl] sulfone), showed signs of hypoxic cell-sensitizationin vivowhen given systemically by intraperitoneal injections. In addition, preliminary testing of the nitrobenzene NDPP (p-nitro-3-dimethyl-propriophenone hydrochloride) indicated that when it was injected directly into the tumour and irradiation was completed within ten minutes after injection, appreciable sensitization was obtained. More detailed studies indicated that both metronidazole at 1,500 mg/kg and tinidazole at 750 mg/kg given intraperitoneally gave an enhancement ratio of 1·5 for a chronically hypoxic cell population in this solid tumour in air-breathing mice. Measures of plasma levels of metronidazole and enhancement ratios obtained in the presentin vivosystem seem in relative agreement with thein vitroandin vivoresults of others.