1. Synthetic hematoporphyrin hydrochloride when injected into experimental animals (white rat and frog) produces photosensitization and motor activation with evidences of skin irritation.2. When injected in test doses intradermally in human beings it produces marked local photosensitization and residual pigmentation after exposure to sunlight.3. The changes in body metabolism and blood chemistry of 12 patients do not seem to be significant.4. When administered either orally or intramuscularly or by both techniques to psychotic patients it seems to produce activation, in some cases mental stimulation and in others general somatic improvement. In 55 cases of affective reactions treated 36.4 per cent showed marked sustained improvement, 18.2 per cent moderate clinical improvement, 18.2 per cent general physical improvement but no change in the course of the psychosis, while 27.2 per cent were not benefited.5. The activation in some instances cannot be regarded as a good therapeutic result.6. The specific effects of its photosensitizing power seem to be made evident in the sensitization of the skin to sunlight and in at least one case in photophobia.7. It is suggested that the beneficial physiologic and psychologic actions of the drug are not necessarily due to some poorly understood changes in the vegetative nervous system resulting from skin sensitization. They may be due to the improved oxygen uptake of the brain cells. This is greatly enhanced in the presence of light and by the increased sensitization of the patient to light.Abstract Teaser