Hypericism, a light sensitivity occurring in domestic animals following ingestion of Hypericum perforatum (St. Johnswort), was studied experimentally to determine the etiology of the disease. Hypericin, a red fluorescent pigment of the plant, was identified spectroscopically in extracts of various tissues from rabbits sensitized by feeding on H. perforatum. Hypericin was also demonstrated spectroscopically in the skin of a living rabbit sensitized in like fashion. Oral adm. of hypercin caused typical photo-sensitization in white rats. The region of the visible spectrum chiefly responsible for hypericism, 540 m[mu] to 610 m[mu], corresponded within reasonable limits to the principal spectral absorption bands of hypericin solns. Hypericism is evidently caused by ingestion of the hypericin present in the plant. Subsequent distribution of the pigment in the skin together with exposure to light brings on the typical syndrome of the disease.