Polycythemia and anemia were induced in a patient with congenital erythropoietic porphyria as a possible means of altering erythropoiesis and its attendant porphyrin production. Maintenance of hematocrits at 50 per cent and 60 per cent for periods of 2 weeks decreased both erythropoiesis and porphyrin excretion to about one-half of their initial levels. Conversely with a hematocrit of 25, erythropoiesis and porphyrin production were increased to about twice the basal level. Examination of the marrow showed fluorescence in all late normoblasts and indicated that the patient has only one red cell population. During induced polycythemia, a shift in marrow erythroid population to relatively mature forms, apparently related to delayed enucleation, was accompanied by a retention of the reticulocyte pool and fluorescent cells within the marrow. During anemia, the occurrence of relatively more immature erythroid cells was accompanied by a shift of the reticulocyte pool and fluorescent cells into the circulating blood. During polycythemia, increases were observed in the relative number of nuclear vacuoles peculiar to this disease, in the retention of radioiron in the marrow and in the relative amount of stercobilin excretion. In anemia these parameters decreased towards normal.