The effects of photodynamic therapy (PDT) on normal brain tissue and depth of brain necrosis were evaluated in rats receiving 2.5 mg/kg aluminum phthalocyanine tetrasulfonate. Twenty-four hours later brains were irradiated with 675 nm light at a power density of 50 mW/cm2 and energy doses ranging from 1.6 to 121.5 J/cm2. Brains were removed 24 h after PDT and evaluated microscopically. When present, brain lesions consisted of well-demarcated areas of coagulation necrosis. When plotting the depth of necrosis against the natural log of energy dose, the data fit a piecewise linear model, with a changepoint at 54.6 J/cm2 and an x intercept of 7.85 J/cm2. The slopes before and after the changepoint were 2.04 and 0.21 mm/ln J cm-2, respectively. The x intercept suggests a minimum light dose below which necrosis of normal brain will not occur, whereas the changepoint indicates the energy density corresponding to an approximate maximum depth of necrosis.