Three patients with porphyria hepatica are reported including I with the mixed type and 2 with porphyria cutanea tarda. The administration of chloroquine, in total doses ranging from 1 to 2.5 gm., resulted in an exacerbation of clinical symptoms by the third or fourth day, manifested by malaise, fever, tachycardia, and impaired liver function. In addition, a considerable disturbance of urinary porphyrins occurred, which was estimated by quantitative studies.