The effect of radiant energy (λ>320 mμ) upon the porphyrin metabolism of normal and hexachlorobenzene-fed porphyric rats was studied. Radiant energy did not alter urinary and fecal porphyrin excretion in normal rats, whereas, in some porphyric animals increase in the excretion of all the urinary and fecal porphyrin fractions was observed. Only uroporphyrin could be detected in the skin of normal and porphyric rats. Porphyric rats had a consistently higher cutaneous uroporphyrin content than the normal. Light exposure resulted in lowering the cutaneous uroporphyrin content of all animals. Liver porphyrins increased in light-restricted porphyric animals receiving hexachlorobenzene. The uroporphyrin content of liver diminished in both the control and test animals during light exposure. Results are compared with the findings obtained in a study of the urinary and fecal porphyrin excretion by patients with mixed or hepatic porphyria published elsewhere.3 Hypotheses are offered to explain these light-evoked alterations in porphyrin metabolism and their possible role in the pathogenesis of the photosensitivity in porphyria.