In vitro and in vivo experiments were performed to evaluate the parameter of light dose rate as it relates to the efficiency of hematoporphyrin derivative (HPD)-induced photosensitization. Exponentially growing Chinese hamster ovary cells were incubated with HPD (25 micrograms/ml) and were then exposed to red light (630 nm) delivered at different dose rates. A total of five dose rates (0.5, 5.0, 15, 23, and 60 milliwatts/sq cm) were examined following a 1-hr HPD incubation, two dose rates (1 and 20 milliwatts/sq cm) were examined after a 12-hr HPD incubation, and three dose rates (0.4, 4, and 40 milliwatts/sq cm) were examined following a 16-hr incubation and a 30-min serum wash protocol. The effect of light dose rate was determined from cell survival curves obtained by standard clonogenic colony formation assays. Similar levels of cellular toxicity were obtained when cells from each HPD incubation group were treated with equal doses of red light delivered at different dose rates. For in vivo experiments, albino mice were given injections of HPD (7.5 mg/kg) and 24 h later the right hind leg of each mouse was treated with localized red light (630 nm). A total dose of 270 J/sq cm was delivered to the right hind leg at dose rates of 5, 25, or 125 milliwatts/sq cm. The resulting acute skin damage induced by HPD photosensitization was scored over a 30-day period, and skin response curves for the three dose rates were obtained. Comparable levels of damage were induced in each of the three experimental groups. The results obtained from both in vitro and in vivo studies indicate that the photosensitizing efficiency of HPD photodynamic therapy is not affected by nonthermal variations in clinically relevant dose rates of delivered light.