We sought to evaluate whether radiosurgery induces apoptosis in an experimental glioma model and to elucidate the time course of this radiobiologic phenomenon. Fischer 344 rats harboring established intracranial 9L gliosarcomas underwent radiosurgery (n = 42) or no radiosurgery (n = 45). Animals were sacrificed at 3, 6, 12, 24, 48, 72 h, and 1 or 2 weeks after treatment and in situ tumor apoptosis was assessed by specific staining. Tumor apoptosis was noted to be statistically higher in radiosurgery-treated animals relative to controls at the 6-, 24-, and 48-hour time points following radiosurgery. Radiosurgery induces apoptosis in the rat intracranial 9L gliosarcoma in a time-dependent fashion. The time course of this radiobiologic phenomenon begins at approximately 6 h following radiosurgery, continues up to 48 h, and begins to decline by 72 h.