The 9L gliosarcoma, grown subcutaneously in juvenile Fischer 344 rats, was studied by in vivo 31P NMR spectroscopy following treatment with 1,3-bis(2-chloroethyl)-1-nitrosourea. Dose-dependent increases in the proportion of high-energy phosphates were observed for doses between 10 and 36 mg/kg (from 80% of the LD10 to greater than the LD50). These doses reduced clonogenic cell survival in a dose-dependent fashion by as much as 3 log orders and resulted in up to 16 days of growth delay (to pretreatment tumor volume). Increases in high-energy phosphates (relative to Pi) in the tumor were greater at higher doses despite the higher levels of clonogenic cell killing and the substantial host systemic toxicity.