Renal function was studied in patients with laryngeal cancer during radiation therapy performed under the conditions of mean fractionation in the presence of metronidazole radiosensitization up to the summary focal dose of 20 Gy. As compared to the findings before treatment an increase in the level of residual nitrogen, urea, uric acid and a decreased amount of creatinine in the plasma were noted. Decreased clearance of the urea, elevated Ambard's urosecretory constant were observed. The nature of these shifts was preserved even after the termination of subsequent radiation therapy up to the summary focal dose of 32 Gy without using the radiosensitogen. There is a possibility of a direct toxic effect of metronidazole on the renal tissue regardless of the state of oxygenation.