Hematoporphyrin derivative (HpD) is a systemically administered photosensitizing agent that may be of value in the treatment of solid tumors. When corneal endothelial cells were perfused in the specular microscope with HpD and exposed to a 25-W incandescent light at 5 cm (5.5 mW/cm2) there was anatomic disruption of corneal endothelial cells and swelling of the corneal stroma. Perfusion with 0.2 microliter/ml (1.0 microgram/ml) HpD and 5 min exposure to light resulted in a corneal swelling of 71 +/- 4 microns after 3 hr, whereas perfusion with 0.2 microliter/ml HpD and a 1-min exposure to light resulted in a corneal swelling of 36 +/- 4 microns after 3 hr. Perfusion with 0.2 microliter/ml HpD with no light exposure resulted in a corneal swelling of 22 +/- 4 microns after 3 hr. Inclusion of 100 micrograms/ml catalase in the perfusion solution resulted in a significant 38% reduction of the corneal swelling. The inclusion of either 100 micrograms/ml superoxide dismutase, 15 mM D-mannitol, 5 mM ascorbic acid, 1/4% DMSO, 50 microns EDTA, 50 microns DETAPAC, 10 mM L-histidine, or 1 mM sodium azide did not modify the corneal swelling induced by the photosensitization reaction. Perfusion of corneal endothelial cells with 2 microliters/ml (10 micrograms/ml) HpD and exposure to 25-W incandescent light for 5 min resulted in swelling of mitochondria, the appearance of vacuoles in the cytoplasm, and rapid corneal swelling. The data suggests that corneal endothelial cells can be damaged by hydrogen peroxide generated by the dismutation of superoxide anion produced during the photoreaction. Superoxide anion itself and hydroxyl-free radical do not appear to participate in causing the endothelial cell damage. The role of singlet oxygen remains somewhat unclear. The data suggests that further in vivo studies should be performed to delineate precautions that should be taken to protect the corneal endothelium during photoradiation therapy.