Nitroxide compounds are stable free radicals which were previously investigated as hypoxic cell radiosensitizers. The stable nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol) has recently been shown to protect aerated cells in culture against superoxide generated from hypoxanthine/xanthine oxidase, hydrogen peroxide, and radiation-induced cytotoxicity and to modestly sensitive hypoxic cultured cells. To extend these observations from the cellular level to the whole animal, the toxicity, pharmacology, and in vivo radioprotective effects of Tempol were studied in C3H mice. The maximum tolerated dose of Tempol administered i.p. was found to be 275 mg/kg, which resulted in maximal Tempol levels in whole blood 5-10 min after injection. Mice were exposed to whole-body radiation in the absence or presence of injected Tempol (275 mg/kg) 5-10 min after administration. Tempol treatment provided significant radioprotection (P less than 0.0001); the dose of radiation at which 50% of Tempol-treated mice die at 30 days was 9.97 Gy, versus 7.84 Gy for control mice. Tempol represents a new class of in vivo, non-sulfur-containing radiation protectors. Given the potential for hypoxic radiosensitization and aerobic cell radioprotection, Temporal or other analogues may have potential therapeutic application.