Brain and glioma tissue levels of tritiated hematoporphyrin derivative (3H-HPD) were measured in normal and 9L intracerebral glioma-bearing rats at 24 hours following administration of 3H-HPD 2-20 mg/kg and at 24-120 hours after 3H-HPD 10 mg/kg. Levels of 3H-HPD in blood, liver, spleen and muscle were also measured. Tissue levels of 3H-HPD increased progressively as the dose was increased. In animals given 10 mg/kg, gradual decreases in tissue levels occurred between 24 and 72 hours but thereafter remained stable. The 3H-HPD level in gliomas was consistently 2-3 X greater than in brain tissue, despite changes in dosage and time interval. High levels of activity were measured in normal brain tissue at all dosage levels, and subsequent clearance of the 3H-HPD from brain, glioma, and other tissues was slow; at 120 hours after administration of 10 mg/kg, approximately 50% of the 24 hour level was still present. These results indicate that although a dose- and time-independent preferential uptake of hematoporphyrin derivative occurs in intracerebral gliomas, persistent high levels may be present in the surrounding brain. The disadvantages of using hematoporphyrin derivative rather than its individual components in studies of HPD uptake and photosensitization in the brain are discussed.