Erythropoietin (Ep) levels were evaluated in serum of neonate, weanling, or adult rats subjected to 1) sham operation, nephrectomy, and/or subtotal hepatectomy and 2) a standard bout of hypoxia (0.45 atm air/6 h, starting 1 h after the operation). Ep activity was quantitated by means of strictly controlled assays in exhypoxic polycythemic mice. The sum of Ep titers in the serum of nephrectomized or hepatectomized rats was compared to Ep levels in sham-operated animals of corresponding age levels, with the exception of 1-wk-old rats: it is relevance that no significant difference is apparent between these Ep production curves. Thus, evidence is presented indicating for the first time that Ep derives from two functionally distinct and additive sources, i.e., the kidney and the liver. Liver Ep, although prevalent in neonatal animals, is obscured in the weanling adult rat by both gradual initiation of massive renal Ep production and progressive decrease of hepatic Ep activity.