Knowledge regarding the in vivo metabolic fate of the glycoprotein erythropoietin (EPO) is incomplete. To determine whether EPO pharmacokinetics are perturbed by ablation of the kidneys or liver or by anesthesia, EPO pharmacokinetic parameters were determined in adult sheep. Animals were studied in a paired manner, with and without deep barbiturate anesthesia and before and immediately after nephrectomy or hepatectomy accompanied by deep barbiturate anesthesia. Hepatectomy was accomplished with the liver left in situ, by occlusion of the arterial hepatic blood supply and diversion of portal venous flow to the jugular vein. After i.v. administration of tracer amounts of 125I-labeled recombinant human EPO, multiple blood samples were taken over 6 to 7 hr and analyzed for EPO immunoprecipitable radioactivity. EPO pharmacokinetic parameters were derived using a noncompartmental system analysis applied to the data on EPO immunoprecipitable radioactivity. No significant differences were detected for plasma clearance, distribution volume, mean residence time and alpha and beta half-lives examined under each of the three paired study conditions. Contrary to speculation by others, results of the present study make it highly unlikely that removal of the terminal sialic acid moieties of EPO contributes significantly to the metabolism of EPO. Because removal of the liver and kidney had no effect on EPO elimination, the metabolic degradation of EPO occurs in a tissue compartment that is yet to be defined. We speculate that the bone marrow is the most likely tissue with primary responsibility for the metabolism of EPO.