The transplantable Guerin epithelioma in Wistar rats was used to test the in vivo effectiveness of 1-(2-hydroxy-3-methoxypropyl)-2-methyl-4-nitroimidazole (P1) as a tumour-cell radiosensitizer after its oral administration at relatively low doses. The radiosensitizing ability of P1 was compared with that of metronidazole. The results indicate that P1 is less toxic than metronidazole, and greater concentrations of P1 in blood and tumour tissues are obtained for the same administered dose of the compounds. The radiosensitizing ability of P1, determined from tumour-regression rates and local-control percentage at 130 days, was higher than that of metronidazole.