Abstract: This study was designed to evaluate the effects of irradiation on the peameability of mice brain to methotrexate (MTX). MTX was administered by intraperitoneal injection (10mg/kg) to unirradiated mice, and to mice receiving varying doses of cranial irradiation. 30 minutes after injection, MTX concentrations in brain and in blood were detected using phospho reseence method. There was a significant increase of MTX level in brain tissue after a single dose irradiatiOn (≥20Gy). Fractionated irradiation also increased MTX level in brain tissue. The effect of irradiation on brain drug permeability could be enhanced by hematoporphyrin derivative (HPD) given 30 minutes before irradiation.