The successful application of phototherapy to subcutaneous tumors has suggested that a similar procedure should be developed for treating gliomas. As a result, attempts are being made to determine a set of conditions that would optimize the destruction of tumor cells while minimizing injury to surrounding brain tissue. To initiate this task, we developed a novel assay method to assess the amount of phototoxicity induced in normal brain by light exposure of mice treated with hematoporphyrin derivative (HPD). The application of this procedure demonstrated that a sufficient amount of HPD was retained in brain tissue, even 72 hours after injection, to cause severe cerebral damage in light-treated mice.