Photoirradiation treatment depends on exposing tumors to a photosensitizer and light to achieve selective tumor kill. We evaluated the kinetics of uptake of a photosensitizer, hematoporphyrin derivative (HpD), in an animal model of cerebral glioma to ascertain the optimal time for photoirradiation therapy. Animal models of cerebral glioma were established by implanting cells from the rat C6 glioma cell line into rat brains or as xenografts in adult mouse brains. C6 cells (107) injected into the frontal lobe of adult Wistar rats produced intracranial tumors greater than 5 mm in diameter in 90% of the animals at 21 days. Tumors greater than 4 mm in diameter developed in adult mouse brains within 14 days after 106 cells were implanted into the frontal lobe. These two tumor models were used to investigate the localization of HpD. After HpD administration, its presence was detected in fresh, unfixed specimens by fluorescence emission after excitation with an ultraviolet lamp. Fluorescence was determined quantitatively by an image analysis method using an optical data digitometer. The fluorescence, which was highly localized selectively to the intracerebral tumor, was just detectable 5 minutes after an intravenous injection of HpD. Patchy, bright fluorescence was evident 4 hours after injection, and the tumor was uniformly fluorescent after 6 hours. A minimal dose of 0.5 mg of HpD per kg of body weight was necessary to produce detectable fluorescence, and the dose of HpD necessary to produce detectable fluorescence was 4 mg/kg of body weight. The intracarotid route of administration was unsatisfactory because seizures were induced, and intrathecal injection did not produce significant fluorescence in the tumor. Fluorescence was detected in the normal brain only in areas outside the blood-brain barrier or when the blood-brain barrier had been disrupted by intracarotid mannitol infusion. In a parallel study, the C6 glioma cells were harvested from mouse brains and cultured in vitro after HpD exposure. These cells were more sensitive to photoirradiation than fibroblast cells. We concluded that HpD localizes rapidly in C6 cerebral glioma because the blood-brain barrier is disrupted; HpD does not cross an intact blood-brain barrier. The mechanism of selective uptake of HpD into cerebral gliomas seems to be different from its selective retention by extracranial tumors. The C6 glioma model is useful for studying photoirradiation therapy in vivo and in vitro.