The acute effects of photoradiation after administering hematoporphyrin derivative (Hpd) on capillary blood flow were studied in intrahepatic tumors and normal liver. The tumors were solitary Walker carcinosarcomas implanted within the livers of Sprague-Dawley rats. Capillary flow was measured by a laser doppler monitor with its probe positioned over the tumor or over normal liver. Within a minute after intraportal Hpd injection (1.7 mg), capillary flow in the tumors began to decrease. Minimal levels of flow were maintained for as long as 15 minutes after Hpd injection with no observed recovery of flow back to control levels. Ratio of minimal flow/control flow averaged 0.36. Similar results were seen in studies on normal liver tissue. These studies demonstrate the extremely rapid vasoactive effects caused by photoradiation of Hpd. Vasoconstriction, vascular stasis and ischemia have proven to be important mechanisms in producing tumor cell destruction by photodynamic therapy.