✓ Photodynamic therapy was studied in dogs with and without posterior fossa glioblastomas. This mode of therapy consisted of intravenous administration of Photofrin-II at doses ranging from 0.75 to 4 mg/kg 24 hours prior to laser light irradiation in the posterior fossa. Tissue levels of Photofrin-II were four times greater in the tumor than in the surrounding normal brain. Irradiation was performed using 1 hour of 500 mW laser light at a wavelength of 630 nm delivered through a fiberoptic catheter directly into the tumor bed via a burr hole. All animals receiving a high dose (4 or 2 mg/kg) of Photofrin-II developed serious brain-stem neurotoxicity resulting in death or significant residual neurological deficits. A lower dose (0.75 mg/kg) of Photofrin-II produced tumor kill without significant permanent brain-stem toxicity in either the control animals or the animals with cerebellar brain tumors receiving photodynamic therapy.