The possible mechanisms of action of cobalt in stimulating erythropoietin production are considered.It is demonstrated that although arsenite shares some ofits biochemical effects with cobalt, it does not stimulate the production of erythropoietin. It has been claimed previously that methylene blue could prevent the development of erythrocytosis after cobalt. In the present series of experiments, however, methylene blue did not affect the production of erythropoietin after injection of cobalt to rabbits.