The chief points presented in this paper are:

1. Following the ingestion of buckwheat (plant or seed) varicolored guinea pigs, white swine and goats exhibited symptoms of photosensitization, the degree of sensitization being in the order given.

2. Rabbits, dogs, white mice and rats did not manifest symptoms of photosensitization.

3. The symptoms and reactions were: agitation, itching, scratching of the ears, weakness, urticaria with sloughing and symptoms similar to those in anaphylaxis.

4. Microscopic examinations showed the lack of marked pathologic change. The lesions, such as petechial hemorrhage of the lungs, brain, liver, stomach and kidneys, suggest that profound toxemia has been present.

5. Lesions were not found which appeared to be suggestive of malignant neoplasms.

6. Irradiation by a quartz mercury vapor lamp apparently develops a resistance to photosensitization, probably because of increased pigmentation induced by ultra-violet light.

7. From the nature of the physiologic and pathologic reactions produced under various filters and from a consideration of the percentages of transmission of solar energy in the visible spectrum, it would seem that the region of photosensitization lies between 580 millimicrons (yellow) and the red end of the spectrum. This conclusion, moreover, is substantiated by the fact that irradiation by a quartz mercury vapor lamp (which radiates no energy in the visible spectrum at a wave-length greater than 579 millimicrons) produces no symptoms or reactions.

8. Spectrophotometric determinations of alcoholic extracts of grass (non-toxic) and of buckwheat (toxic) show the presence of two additional bands in the absorption spectrum of buckwheat with maxima at about 540 and 600 millimicrons, respectively, together with the common absorption zones at 430 to 490 millimicrons and 630 to 690 millimicrons.

9. Spectrophotometric determinations of blood serums of sensitized animals show, besides the usual absorption bands peculiar to oxyhemoglobin (with maxima at 540 and 580 millimicrons respectively), two zones with maxima at 600 and 660 millimicrons respectively.

10. The fluorescence of chlorophyll per se, as suggested by previous investigators, is not, in all probability, the cause of the sensitization induced by buckwheat.

11. Hematoporphyrine is not the photodynamic substance in all probability.

12. Phylloporphyrine may be the photodynamic substance. In this regard, also, the possibility of cholehematin is not to be ruled out.