Among the group of nitroimidazole radiosensitizers, predominantly misonidazole and metronidazole have so far been submitted to an extensive clinical testing. However, neurotoxic side effects have limited the dose of the substance necessary to obtain a maximal effect of sensibilization. Therefore, extensive efforts are being undertaken in the search for new compounds of equally good radiosensitizing action but with less side effects. One approach is to use substances of lower lipophilicity which will reduce penetration into the cerebral tissue. Proceeding from these considerations, comparative investigations were carried out with metronidazole (1-(2'-hydroxyethyl)-2-methyl-5-nitroimidazole) and iso-metronidazole (1-(2'-hydroxyethyl)-2-methyl-4-nitroimidazole), since in vitro but also in animal experiments the both substances showed an equally good radiosensitizing action while the lipophilicity of iso-metronidazole is much lower. Whereas for the metronidazole about the same distribution of the substance was found in all the organs investigated and also in the tissue of the mammary adenocarcinoma of C3H mice, the maximal concentration of iso-metronidazole in the cerebral tissue reached only about 50% of that in tumor tissue. This property, combined with the improved solubility, make the iso-metronidazole appear more appropriate for clinical testing than metronidazole.