The articles featured in the Blood Flashback series were selected and the commentaries written by the Blood Editors. Complete versions of all the articles in the series, commemorating the journal's 70th-anniversary year, can be found on the Blood Web site at www.bloodjournal.org/collection/blood-flashback. In this historic study, Kurt R. Reissmann used parabiotic rats to demonstrate the functional existence of erythropoietin. In a crossed circulation setting, 1 rat was kept in conditions of hypoxia and the other in normal oxygen. Both rats developed reticulocytosis, increased hemoglobin, and bone marrow hyperplasia. He concluded that erythropoiesis is regulated by a humoral factor rather than by a direct oxygen effect on the bone marrow. Following this pivotal work, the role of the kidney in erythropoiesis was shown by Leon Jacobson in 1957 and erythropoietin was eventually purified from the urine of anemic patients by Eugene Goldwasser in 1977. Parabiotic experiments are still used and were recently applied to investigate aging and rejuvenation related to systemic factors.