During a study of photodynamic action in white mice we became aware of the effect of heat on this process. A review of the literature revealed that little work had been done regarding this problem, and available reports provided conflicting results. In an effort to resolve these differences, we decided to study the influence of heat on the photodynamic process in white mice. Materials and Methods Photosensitizer.— The acetic acid-sulfuric acid derivative of hematoporphyrin hydrochloride was used as in previous experiments because of the work of Schwartz, alone9 and with others,10,11 who showed the commercial hematoporphyrin to be a crude mixture of many porphyrins. Solutions containing 10 mg. per milliliter were made up and stored in brown bottles under refrigeration. Animals.— White BALB male mice, 6 to 7 weeks old, were used in the experiments.* The photosensitization, light, temperature, dark box, and animal wheel used in this study