Laboratorio de Hematología Experimental, Cátedra de Fisiología, Facultad de Odontología, Universidad Nacional de Buenos Aires, M. T. de Alvear 2142, Buenos Aires, Argentina (Received 16 January 1976) The importance of the kidney in the production of erythropoietin (erythropoiesis-stimulating hormone, ESH) in rodents has been demonstrated by a number of investigators (Krantz & Jacobson, 1970) since the initial observations of Jacobson, Goldwasser, Fried & Plzak (1957). The results of several studies indicate, however, that ESH can be produced in extrarenal sites after bilateral nephrectomy (Fried, Kilbridge, McDonald & Lange, 1969; Schooley & Mahlmann, 1972a), Erythropoietin activity is found in plasma of anephric rodents (Fried et al. 1969; Schooley & Mahlmann, 1972a) and man (Mirand, Murphy, Steeves, Weber & Retief, 1968; Naets & Wittek, 1968) when they are intensely stimulated, and this is due to production of ESH in extrarenal sites rather than release of preformed ESH from storage sites (Schooley &