Light activation of circulating hematoporphyrin derivatives has been used in the treatment of selected brain tumors. The effects of this photodynamic therapy on the non-neoplastic, adjacent brain tissue are incompletely characterized. We studied in adult Fisher rats the time-dependent (1 hour to 7 days) effects of photoactivated Photofrin II. Our protocol was comparable to that used in the treatment of human brain tumors. Structural and functional changes spread from the treatment surface and from the center to the periphery to involve the entire cerebral cortex exposed under a 5 mm craniectomy. The sequential changes spreading from the surface to the deepest cortical layer involve first astrocytes (1 hour), then endothelial cells and, ultimately, neurons. Thrombi were first noted in the microvasculature after 18 hours and coagulation necrosis of the entire areaat risk occurred only after 48 hours. The results suggest that the photosensitizing agent crosses the intact blood-brain barrier and enters the astrocytic compartment where it becomes cytotoxic upon light activation. A comparison between the focal brain lesions of photodynamic therapy and those induced by middle cerebral artery occlusion suggests that cell damage evolves along different paths in these two forms of brain injury.