A rapid method to determine the systemic incorporation of amifostine has been sought in order to determine the effectiveness of different administration routes without the delay inherent in awaiting therapeutic results. Consistent changes in animal measurements of nitroxide signal decay were monitored using in vivo EPR at frequencies low enough to ensure uniform sensitivity to organs deep in 20-g C3H mice. Conditions included both co-administration of the amifostine with the carbamoyl-proxyl spin probe (CP) via i.p. injection (n=6) and oral administration (n=8) of the amifostine. These decreased the first order rate of decay of the CP EPR signal after a dose of 13.5 Gy radiation, by 23% and 18%, respectively. These changes were significantly different from the rate of decay of the CP EPR signal without amifostine, but were statistically indistinguishable from each other. These data demonstrate: (1) condition-dependent exponential decay of CP EPR signal allowing its use to determine systemic availability of a drug, and (2) that oral administration and i.p. injection of amifostine are both effective in affecting the CP EPR signal decay rate in a mouse model. This is a strong indicator of similar bioavailability in mice from both routes of administration.