Sites of erythrogenin production in the neonatal rat were studied. Five-day-old neonates rendered hypoxic displayed no detectable levels of erythrogenin in their kidneys, but highly elevated amounts of this factor were noted in both their livers and spleens. These heightened levels of hepatic and splenic erythrogenin most likely form the basis for the ability of the bilaterally nephrectomized neonatal rat to show little impairment in EP production in response to hypoxia.